Spinal motion preservation assemblies

ABSTRACT

Spinal motion preservation assemblies adapted for use in a spinal motion segment are disclosed including the process for delivering and assembling the spinal motion preservation assemblies in the spinal motion segment via an axial channel created with a trans-sacral approach. Many of the spinal motion preservation assemblies make use of a dual pivot. A number of different embodiments of spinal motion preservation assemblies are disclosed which include at least one component adapted for elastic deformation under compressive loads. The disclosed mobility preservation assemblies provide for dynamic stabilization of the spinal motion segment. Other variations and implementations of the teachings are disclosed, including the sheathed delivery of membranes in order to protect the membranes before and during deployment.

BACKGROUND OF THE INVENTION

This application builds upon a series of applications filed on behalf ofassignee. In particular this application extends the innovative work inthe area of spinal motion preservation assemblies described inco-pending and commonly assigned U.S. patent application Ser. No.11/256,810 for Spinal Motion Preservation Assemblies and U.S. patentapplication Ser. No. 11/259,614 Driver Assembly for Simultaneous AxialDelivery of Spinal Implants. This application claims priority andincorporates by reference both the '810 application and the '614application. This application claims priority and incorporates byreference two provisional applications claimed as priority documents bythe '810 application specifically, U.S. Provisional Application No.60/621,148 filed Oct. 22, 2004 for Spinal Mobility PreservationAssemblies and U.S. Provisional Application No. 60/621,730 filed Oct.25, 2004 for Multi-Part Assembly for Introducing Axial Implants into theSpine. This application claims priority and incorporates by referencefour co-pending and commonly assigned U.S. patent application Ser. Nos.10/972,184, 10/972,039, 10/972,040, and 10/972,176 all filed on Oct. 22,2004. These four applications claim priority to two United StatesProvisional Applications: Application No. 60/558,069 filed Mar. 31, 2004and Application No. 60/513,899 filed Oct. 23, 2003. Priority to thesetwo provisionals is claimed through the four co-pending applications andthe provisionals are incorporated by reference. This application alsoclaims priority through the '810 application to U.S. patent applicationSer. No. 11/199,541 filed Aug. 8, 2005 and U.S. Provisional ApplicationNo. 60/599,989 filed Aug. 9, 2004 which is claimed as a prioritydocument for the '541 application. Both of these applications areincorporated by reference.

This application incorporates by reference a set of United Statesapplications, provisional applications, and issued patents including:60/182,748 filed Feb. 16, 2000; Ser. No. 09/640,222 filed Aug. 16, 2000(now issued as U.S. Pat. No. 6,575,979); 10/459,149 filed Jun. 11, 2003;Ser. No. 09/684,820 filed Oct. 10, 2000 (now issued as U.S. Pat. No.6,558,386); 10/430,751 filed May 6, 2003; 60/182,748 filed Feb. 16,2000; Ser. No. 09/782,583 filed Feb. 13, 2001 (issued as U.S. Pat. No.6,558,390); 09/848,556 filed May 3, 2001 (now issued as U.S. Pat. No.7,014,633); 10/125,771 filed Apr. 18, 2002 (issued as U.S. Pat. No.6,899,716); 10/990,705 filed Nov. 17, 2004; Ser. No. 10/430,841 filedMay 6, 2003; Ser. No. 09/710,369 filed Nov. 10, 2000 (issued as U.S.Pat. No. 6,740,090); 10/853,476 filed May 25, 2004; Ser. No. 09/709,105filed Nov. 10, 2000 (issued as U.S. Pat. No. 6,790,210); 09/782,534filed Feb. 13, 2001; application Ser. Nos. 10/971,779, 10/971,781,10/971,731, 10/972,077, 10/971,765, 10/972,065, 10/971,775, 10/971,299,10/971,780, all filed Oct. 22, 2004; 60/706,704 filed Aug. 9, 2005; Ser.No. 11/189,943 filed Jul. 26, 2005, Ser. No. 10/309,416 now U.S. Pat.No. 6,921,403 filed Dec. 3, 2002. While these applications have beenincorporated by reference to provide additional detail it should benoted that these other applications (including those that havesubsequently issued as patents) were written at an earlier time and hada different focus from the present application. Thus, to the extent thatthe teachings or use of terminology differ in any of these incorporatedapplications from the present application, the present applicationcontrols.

FIELD OF THE INVENTION

The present invention relates generally to implantable deviceassemblies, instrumentation systems, and methods for accessing andtreating a spinal motion segment via various access routes including aminimally-invasive trans-sacral approach (as described in U.S. Pat. No.6,558,390 which is incorporated herein by reference) and procedures forthe deployment of implantable components and assemblies that areanchored in bone that can be used to distract, decompress, and stabilizewhile preserving motion in vertebral motion segments in the human spineto relieve lower back pain, restore physiological function of the lumbarspine, and prevent progression or transition of degenerative disease.More specifically, the present disclosure generally relates to spinalmotion preservation assemblies (MPA) including assemblies adapted to beintroduced percutaneously through tissue to an access point on the spinein a minimally invasive, low trauma manner, to provide therapy to thespine.

BACKGROUND OF THE INVENTION Overview

The present invention is an extension of work in a series of patentapplications (some now issued patents) with a common assignee. Much ofthe work is described in great detail in the many applicationsreferenced above and incorporated by reference into this application.Accordingly, the background of the invention provided here does notrepeat all of the detail provided in the earlier applications, butinstead highlights how the present invention adds to this body of work.

The spinal column is a complex system of bone segments (vertebral bodiesand other bone segments) which are in most cases separated from oneanother by discs in the intervertebral spaces (sacral vertebrae are anexception). FIG. 1 shows the various segments of a human spinal columnas viewed from the side. In the context of the present disclosure, a“motion segment” includes adjacent vertebrae, i.e., an inferior and asuperior vertebral body, and the intervertebral disc space separatingsaid two vertebral bodies, whether denucleated space or with intact ordamaged spinal discs. Unless previously fused, each motion segmentcontributes to the overall flexibility of the spine to flex to providesupport for the movement of the trunk and head.

The vertebrae of the spinal cord are conventionally subdivided intoseveral sections. Moving from the head to the tailbone, the sections arecervical 104, thoracic 108, lumbar 112, sacral 116, and coccygeal 120.The individual vertebral bodies within the sections are identified bynumber starting at the vertebral body closest to the head. Thetrans-sacral approach is well suited for access to vertebral bodies inthe lumbar section and the sacral section. As the various vertebralbodies in the sacral section are usually fused together in adults, it issufficient and perhaps more descriptive to merely refer to the sacrumrather than the individual sacral components.

It is useful to set forth some of the standard medical vocabulary beforegetting into a more detailed discussion of the background of the presentinvention. In the context of the this discussion: anterior refers to infront of the spinal column; (ventral) and posterior refers to behind thecolumn (dorsal); cephalad means towards the patient's head (sometimes“superior”); caudal (sometimes “inferior”) refers to the direction orlocation that is closer to the feet. As the present applicationcontemplates accessing the various vertebral bodies and intervertebralspaces through a preferred approach that comes in from the sacrum andmoves towards the head, proximal and distal are defined in context ofthis channel of approach. Consequently, proximal is closer to thebeginning of the channel and thus towards the feet or the surgeon,distal is further from the beginning of the channel and thus towards thehead, or more distant from the surgeon. When referencing delivery tools,distal would be the end intended for insertion into the access channeland proximal refers to the other end, generally the end closer to thehandle for the delivery tool.

The individual motion segments within the spinal columns column allowmovement within constrained limits and provide protection for the spinalcord. The discs are important to cushion and distribute the large forcesthat pass through the spinal column as a person walks, bends, lifts, orotherwise moves. Unfortunately, for a number of reasons referencedbelow, for some people, one or more discs in the spinal column will notoperate as intended. The reasons for disc problems range from acongenital defect, disease, injury, or degeneration attributable toaging. Often when the discs are not operating properly, the gap betweenadjacent vertebral bodies is reduced and this causes additional problemsincluding pain.

It has been estimated that in 2004 there were over 700,000 surgicalprocedures performed annually to treat lower back pain in the U.S. It isconservatively estimated that in 2004 there were more than 200,000lumbar fusions performed in the U.S., and more than 300,000 worldwide,representing approximately a $1B endeavor in an attempt to alleviatepatients' pain. Approximately 60% of spinal surgery takes place in thelumbar spine, and of that portion approximately 80% involves the lowerlumbar vertebrae designated as the fourth lumbar vertebra (“L4”), thefifth lumbar vertebra (“L5”), and the first sacral vertebra (“S1”).Persistent low back pain is often attributable to degeneration of thedisc between L5 and S1. (See edge between the lumbar region 112 and thesacrum 116 in FIG. 1).

A range of therapies have been developed to alleviate the painassociated with disc problems. One class of solutions is to remove thefailed disc and then fuse the two adjacent vertebral bodies togetherwith a permanent but inflexible spacing, also referred to as staticstabilization. As mentioned above, an estimated 300,000 fusionoperations take place each year. Fusing one section together ends theability to flex in that motion segment. While the loss of the normalphysiologic disc function for a motion segment through fusion of amotion segment may be better than continuing to suffer from the pain, itwould be better to alleviate the pain and yet retain all or much of thenormal performance of a healthy motion segment.

Another class of therapies attempts to repair the disc so that itresumes operation with the intended intervertebral spacing andmechanical properties. One type of repair is the replacement of theoriginal damaged disc with a prosthetic disc. This type of therapy iscalled by different names such as dynamic stabilization or spinal motionpreservation.

The Operation of the Spine

The bodies of successive lumbar, thoracic and cervical vertebraearticulate with one another and are separated by the intervertebralspinal discs. Each spinal disc includes a fibrous cartilage shellenclosing a central mass, the “nucleus pulposus” (or “nucleus” herein)that provides for cushioning and dampening of compressive forces to thespinal column. The shell enclosing the nucleus includes cartilaginousendplates adhered to the opposed cortical bone endplates of the cephaladand caudal vertebral bodies and the “annulus fibrosus” (or “annulus”herein) includes multiple layers of opposing collagen fibers runningcircumferentially around the nucleus pulposus and connecting thecartilaginous endplates. The natural, physiological nucleus includeshydrophilic (water attracting) mucopolysacharides and fibrous strands(protein polymers). The nucleus is relatively inelastic, but the annuluscan bulge outward slightly to accommodate loads axially applied to thespinal motion segment.

The intervertebral discs are anterior to the spinal canal and locatedbetween the opposed end faces or endplates of a cephalad and a caudalvertebral body. The inferior articular processes articulate with thesuperior articular processes of the next succeeding vertebra in thecaudal (i.e., towards the feet or inferior) direction. Several ligaments(supraspinous, interspinous, anterior and posterior longitudinal, andthe ligamenta flava) hold the vertebrae in position yet permit a limiteddegree of movement. The assembly of two vertebral bodies, theinterposed, intervertebral, spinal disc and the attached ligaments,muscles and facet joints is referred to as a “spinal motion segment”.

The relatively large vertebral bodies located in the anterior portion ofthe spine and the intervertebral discs provide the majority of theweight bearing support of the vertebral column. Each vertebral body hasrelatively strong, cortical bone layer forming the exposed outsidesurface of the body, including the endplates, and weaker, cancellousbone in the center of the vertebral body.

The nucleus pulposus that forms the center portion of the intervertebraldisc consists of 80% water that is absorbed by the proteoglycans in ahealthy adult spine. With aging, the nucleus becomes less fluid and moreviscous and sometimes even dehydrates and contracts (sometimes referredto as “isolated disc resorption”) causing severe pain in many instances.The spinal discs serve as “dampeners” between each vertebral body thatminimize the impact of movement on the spinal column, and discdegeneration, marked by a decrease in water content within the nucleus,renders discs ineffective in transferring loads to the annulus layers.In addition, the annulus tends to thicken, desiccate, and become morerigid, lessening its ability to elastically deform under load and makingit susceptible to fracturing or fissuring, and one form of degenerationof the disc thus occurs when the annulus fissures or is torn. Thefissure may or may not be accompanied by extrusion of nucleus materialinto and beyond the annulus. The fissure itself may be the solemorphological change, above and beyond generalized degenerative changesin the connective tissue of the disc, and disc fissures can neverthelessbe painful and debilitating. Biochemicals contained within the nucleusare enabled to escape through the fissure and irritate nearbystructures.

A fissure also may be associated with a herniation or rupture of theannulus causing the nucleus to bulge outward or extrude out through thefissure and impinge upon the spinal column or nerves (a “ruptured” or“slipped” disc). With a contained disc herniation, the nucleus may workits way partly through the annulus but is still contained within theannulus or beneath the posterior longitudinal ligament, and there are nofree nucleus fragments in the spinal canal. Nevertheless, even acontained disc herniation is problematic because the outward protrusioncan press on the spinal cord or on spinal nerves causing sciatica.

Another disc problem occurs when the disc bulges outwardcircumferentially in all directions and not just in one location. Thisoccurs when, over time, the disc weakens bulges outward and takes on a“roll” shape. Mechanical stiffness of the joint is reduced and thespinal motion segment may become unstable, shortening the spinal cordsegment. As the disc “roll” extends beyond the normal circumference, thedisc height may be compromised, and foramina with nerve roots arecompressed causing pain. Current treatment methods other than spinalfusion for symptomatic disc rolls and herniated discs include“laminectomy” which involves the surgical exposure of the annulus andsurgical excision of the symptomatic portion of the herniated discfollowed by a relatively lengthy recuperation period. In addition,osteophytes may form on the outer surface of the disc roll and furtherencroach on the spinal canal and foramina through which nerves pass. Thecephalad vertebra may eventually settle on top of the caudal vertebra.This condition is called “lumbar spondylosis”.

Various other surgical treatments that attempt to preserve theintervertebral spinal disc and to simply relieve pain include a“discectomy” or “disc decompression” to remove some or most of theinterior nucleus thereby decompressing and decreasing outward pressureon the annulus. In less invasive microsurgical procedures known as“microlumbar discectomy” and “automated percutaneous lumbar discectomy”,the nucleus is removed by suction through a needle laterally extendedthrough the annulus. Although these procedures are less invasive thanopen surgery, they nevertheless suffer the possibility of injury to thenerve root and dural sac, perineural scar formation, re-herniation ofthe site of the surgery, and instability due to excess bone removal. Inaddition, they generally involve the perforation of the annulus.

Although damaged discs and vertebral bodies can be identified withsophisticated diagnostic imaging, existing surgical interventions andclinical outcomes are not consistently satisfactory. Furthermore,patients undergoing such fusion surgery experience significantcomplications and uncomfortable, prolonged convalescence. Surgicalcomplications include disc space infection; nerve root injury; hematomaformation; instability of adjacent vertebrae, and disruption of muscle,tendons, and ligaments, for example.

Several companies are pursuing the development of prosthesis for thehuman spine, intended to completely replace a physiological disc, i.e.,an artificial disc. In individuals where the degree of degeneration hasnot progressed to destruction of the annulus, rather than a totalartificial disc replacement, a preferred treatment option may be toreplace or augment the nucleus pulposus, involving the deployment of aprosthetic disc nucleus. As noted previously, the normal nucleus iscontained within the space bounded by the bony vertebrae above and belowit and the annulus fibrosus, which circumferentially surrounds it. Inthis way the nucleus is completely encapsulated and sealed with the onlycommunication to the body being a fluid exchange that takes placethrough the bone interface with the vertebrae, known as the endplates.

The hydroscopic material found in the physiological nucleus has anaffinity for water (and swells in volume) which is sufficiently powerfulto distract (i.e., elevate or “inflate”) the intervertebral disc space,despite the significant physiological loads that are carried across thedisc in normal activities. These forces, which range from about 0.4× toabout 1.8× body weight, generate local pressure well above normal bloodpressure, and the nucleus and inner annulus tissue are, in fact,effectively avascular.

The existence of the nucleus as a cushion (e.g., the nucleus is the“air” in the “tire” known as a spinal disc), and the annulus, as aflexible member, contributes to the range of motion in the normal disc.Range of motion is described in terms of degrees of freedom (i.e.,translation and rotation about three orthogonal planes relative to areference point, the instantaneous center of rotation around thevertical axis of the spine). The advantages of spinal motionpreservation assemblies of the present disclosure in preserving,restoring, and/or managing mobility in terms of flexion, extension,compression, left/right (L/R) rotation, L/R lateral bending, anddistraction will become more apparent from the description of therelationship between movement and anatomical structures of the spine,and the consequences and impact of injury (e.g., trauma/mechanicalinjury or aging) noted below.

Flexion and Extension

Flexion and extension of the spine combine forward sliding and rotationof the vertebrae. The facet joints and the annulus resist the forwardsliding. Rotation is resisted by the annulus; capsules of the facetjoints; action of the back muscles, and passive tension generated by thethoracolumbar fascia. Extension is resisted by the facet joints, andsecondarily by the annulus.

The spine is resistant to injury if the force is only in pure flexion,as the combination of the facet joints and disc are intrinsically stablein this plane. While the spinal muscles can be injured during forcefulflexion since they are important in controlling this motion, ensuingpain is not typically chronic.

Extension is impaired by impaction of the facet joints and eventuallythe inferior articular process against the lamina. This can result in acartilage injury of the facet joint, disruption of the facet capsule andfacet joint, or pars interarticularis fracture.

Compression

Compression of the spine is due to body weight and loads applied to thespine. Body weight is a minor compressive load. The major compressiveload on the spine is produced by the back muscles. As a person bendsforward, the body weight plus an external load must be balanced by theforce generated by the back muscles. That is, muscle loads balancegravitational loads so that the spine is in equilibrium to help keep usfrom falling over. The gravitational load to be offset may be calculatedby multiplying the load times the perpendicular distance of the loadfrom the spine. The greater the distance from the spine, the larger theload is. Since the back muscles act close to the spine, they must exertlarge forces to balance the load. The force generated by the backmuscles results in compression of spinal structures.

Most of the compressive loads (˜80%) are sustained by the anteriorcolumn (disc and vertebral body). The disc is a hydrostatic system. Thenucleus acts as a confined fluid within the annulus. It distributescompressive forces from the vertebral end plates (axial loads) intotension on the annulus fibers.

Compression injuries occur by two main mechanisms; axial loading bygravity or by muscle action. Gravitational injuries result from a fallonto the buttocks while muscular injuries result from severe exertionduring pulling or lifting. A serious potential consequence of the injuryis a fracture of the vertebral end plate. Since the end plate iscritical to disc nutrition, an injury can change the biochemical andmetabolic state of the disc. If the end plate heals, the disc may sufferno long-term consequences. However, if the end plate does not heal, thenucleus can undergo harmful changes. The nucleus may lose itsproteoglycans and thus its water-binding capacity. The hydrostaticproperties of the nucleus may be compromised. Instead of sharing theload between the nucleus and the annulus, more of the load istransferred to the annulus. The annulus fibers may then fail. Inaddition to annular tears, the layers of the annulus may separate(delaminate). The disc may collapse or it may maintain its height withprogressive annular tearing. If the annulus is significantly weakened,there may be a rupture of the disc whereby the nuclear material migratesinto the annulus or into the spinal canal causing nerve rootcompression.

Rotation

Rotation of the spine is accomplished by the contraction of theabdominal muscles acting through the thorax and the thoracolumbarfascia. There are no primary muscles responsible for lumbar rotation.The facet joints and the collagen fibers of the annulus resist thisrotation. In rotation, only 50% of the collagen fibers are in tension atany time, which renders the annulus susceptible to injury.

The spine is particularly susceptible to injury in a loading combinationof rotation and flexion. Flexion pre-stresses the annular fibers. As thespine rotates, compression occurs on the facet joint surfaces of thejoint opposite the rotation. Distraction occurs on the facet joint onthe same side of the rotation. The center of rotation of the motionsegment shifts from the back of the disc to the facet joint incompression. The disc shifts sideways and shear forces on the annularfibers are significant. Since the annular fibers are weak in thisdirection, they can tear. If the rotation continues, the facet jointscan sustain cartilage injury, fracture, and capsular tears while theannulus can tear in several different ways. Any of these injuries can bea source of pain.

Lateral Bending

Bending is a combination of lateral flexion and rotation through theannulus and facet joints.

Distraction

Pure distraction rarely occurs and is usually a combination of tensionand compression on the spinal joints depending on the direction ofapplied force. An example of a distraction force is therapeutic spinaltraction to “unload” the spine.

In the context of the present disclosure, as used herein the termdistraction refers procedurally to an elevation in height that increasesthe intervertebral disc space resulting from introduction of the motionpreservation assembly or prosthetic nucleus device (“PND”), which may beachieved either in the axial deployment of the device itself, orassisted by means of a temporary distraction during the implantationprocedure. Temporary distraction refers to elevation of disc height bymeans, such as a distraction device, which is subsequently removed butwherein the elevation is retained intra-operatively, while the patientremains prone. Thus, the device may be inserted into an elevated discspace first created by other distraction means, and thereafter physicalpresence and dimensionality of the inserted device is key to preservingthat height space, to decompress the disc and alleviate pain caused bynerve impingement.

Thus, if one takes a reference point at the top face of a vertebralbody, the motion segment includes that vertebral body, the next mostcephalad vertebral body, and the intervertebral disc between those twovertebral bodies has six degrees of freedom. If the X axis is alignedwith the anterior/posterior direction, and the Y axis is aligned withthe right and left side, and the Z axis aligned with the cephalad/caudaldirection (sometimes called the cephalad/caudal axis) then the sixdegrees of freedom are as follows:

Translation X axis Movement of upper vertebral body in alonganterior/posterior direction Translation Y axis Movement of uppervertebral body to along right or to left Translation Z axis Movement ofupper vertebral body away along from lower vertebral body (distraction)or towards lower vertebral body compression Rotation in X and Y Rotationof spine clockwise or plane defined by counterclockwise Rotation in Xand Z Rotation of spine to flex or extend plane defined by the spineRotation in Y and Z Rotation of spine to move laterally to plane definedby right or left

To date, drawbacks of currently contemplated or deployed prostheticnucleus devices include subsidence; their tendency to extrude ormigrate; to erode the bone; to degrade with time, or to fail to providesufficient biomechanical load distribution and support. Some of thesedrawbacks relate to the fact that their deployment typically involves avirtually complete discectomy of the disc achieved by instrumentsintroduced laterally through the patient's body to the disc site andmanipulated to cut away or drill lateral holes through the disc andadjoining cortical bone. The endplates of the vertebral bodies, whichinclude very hard cortical bone and help to give the vertebral bodiesneeded strength, are usually weakened or destroyed during the drilling.The vertebral endplates are special cartilage structures that surroundthe top and bottom of each vertebra and are in direct contact with thedisc. They are important to the nutrition of the disc because they allowthe passage of nutrients and water into the disc. If these structuresare injured, it can lead to deterioration of the disc and altered discfunction. Not only do the large laterally drilled hole or holescompromise the integrity of the vertebral bodies, but the spinal cordcan be injured if they are drilled too posterior.

Alternatively, current devices are sometimes deployed through asurgically created or enlarged hole in the annulus. The annulus fibrosusconsists of tough, thick collagen fibers. The collagen fibers which arefound in the annulus fibrosus are arranged in concentric, alternatinglayers. Intra-layer orientation of these fibers is parallel, however,each alternating (i.e., interlayer) layers' collagen fibers are orientedobliquely (˜120). This oblique orientation allows the annulus to resistforces in both vertical and horizontal directions. Axial compression ofa disc results in increased pressure in the disc space. This pressure istransferred to the annulus in the form of loads (stresses) perpendicularto the wall of the annulus. With applied stress, these fibrous layersare put in tension and the angle from horizontal decreases to betterresist the load, i.e., the annulus works to resist these perpendicularstresses by transferring the loads around the circumference of theannulus (Hoop Stress). Vertical tension resists bending and distraction(flexion and extension). Horizontal tension resists rotation and sliding(i.e., twisting). While the vertical components of the annulus' layersenable the disc to withstand forward and backward bending well, onlyhalf of the horizontal fibers of the annulus are engaged during arotational movement. In general, the disc is more susceptible to injuryduring a twisting motion, deriving its primary protection duringrotation from the posterior facet joints; however, this risk is evengreater if and when the annulus is compromised.

Moreover, annulus disruption will remain post-operatively, and present apathway for device extrusion and migration in addition to compromisingthe physiological biomechanics of the disc structure. Other devices, inan attempt to provide sufficient mechanical integrity to withstand thestresses to which they will be subjected, are configured to be so firm,stiff, and inflexible that they tend to erode the bone or becomeimbedded over time in the vertebral bodies, a phenomenon known as“subsidence”, sometimes also termed “telescoping”. The result ofsubsidence is that the effective length of the vertebral column isshortened, which can subsequently cause damage to the nerve root andnerves that pass between the two adjacent vertebrae.

SUMMARY OF THE DISCLOSURE

Spinal mobility preservation assemblies are disclosed which areconfigured to include at least one and often a plurality of pivots orpivot-like components including combinations of components that serve toallow motion in more than one plane, that function in conjunction withone or more elastomeric (e.g., semi-compliant materials capable ofelastic deformation) or spring component (including non-helical springssuch as the relatively flat Belleville disc), that are particularlyeffective in preserving motion in any plane relative to the longitudinalaxis of the spine.

In the context of the present disclosure, “planes” are defined relativeto X, Y, Z orthogonal axes, where Z is the longitudinal axis of thespine. More specifically, rotation about X, Y, Z and motion about X,Yare enabled by use of at least one unconstrained pivot points, and theelastomeric component enables motion in the Z direction and serves todampen axial compression.

While the term pivot is often used in mechanics in reference to apointed shaft forming the center and fulcrum on which something turnsbalances or oscillates, here the term is more like the use of the termin pivot joint (trochoid) in anatomy but is meant to be an even broaderconcept as unlike classic pivots, many of the pivots of the presentdisclosure are able to move with respect with a bearing surface to allowa more complex form of motion. Thus the center of rotation is mobile.Additionally, the cup containing the bearing surface may be free toundergo a limited amount of translation relative to the bone anchorassociated with that cup. Examples are given with respect to the Z axisbut the cup could have a limited ability to move in the X or Y axis.Further, the selective use of a plurality of pivot/bearing surfacecombinations associated with a single bone anchor allow for more complexpivot motions. The use of asymmetric components to allow moretranslation or rotation in some directions than other directions adds tothe ability to support complex pivot movement. The term pivot is alsomeant to include more complicated combinations of components thatprovide an emulation of the same functionality as the pivots disclosedhere.

While the range of motion permitted by a healthy spinal motion segmentvaries from individual to individual, there are typical expected minimumranges of motion for each type of motion for each for each spinal motionsegment. The range of motion for each spinal motion segment is limitednot just by the disc but also by the actions of other protrusions ofbone on the vertebrae and other biological structures. There is no realadvantage to providing a spinal motion preservation assembly thatprovides a range of motion that far exceeds other biological limits onthe ability of the spinal motion segment to move. However, it isdesirable to minimize or eliminate limitations on the motion of a spinalmotion segment attributable to the implanted spinal motion preservationassembly.

As used herein, the term “unconstrained” refers to the fact that pivotsare not fixed, and refers to motion that meets or exceeds the normalrange of motion in all six degrees of freedom. Thus, with respect to thelumbar spine, whereas the normal full range of motion typically allowsfor about 12 degrees of flexion, about 8 degrees of extension, about 9degrees of left or right lateral bend, and about 2 degrees of clockwiseor counterclockwise motion, the mobility preservation assemblies inaccordance with a preferred aspect of the present disclosure (e.g., withunconstrained dual-pivots in conjunction with an elastomeric or a springcomponent) which have been configured for the lumbar spine generallywill permit at least about 4 degrees, often at least about 8 degrees andpreferably no more than about 20 degrees of flexion (bending forwards).For an axis-symmetric device, the device would be capable of the samedegree of movement in flexion, extension (bending backwards), lateralmotion to either side, or motion that is a combination of lateral andextension or lateral and flexion. Rotation is completely unconstrained,with no limitation.

With respect to the spine generally, a motion preservation assembly inaccordance with the present disclosure generally will often provide atleast substantially the normal full range of motion for any particularmotion segment being treated and may provide more than 100% of thenormal full range of motion. A motion preservation assembly createdwithout an elastically compressible component may provide substantiallythe normal range of motion with the exception of the limited amount ofcompression available in the cephalad/caudal axis.

While it is known in the art to implant certain other human joints(e.g., fingers and knees) with devices meant to preserve translation,the spine is the only articulating human joint with six degrees offreedom with respect to motion, as described above. The spinal implantassemblies of the present disclosure are able to preserve motion(including translation) in all six degrees of freedom. The six degreesof freedom when applied to a motion segment can be thought of as theability of one vertebral body to move relative to the other vertebralbody in that motion segment. The spinal implant assemblies of thepresent disclosure are able to preserve motion in all six degrees offreedom because these assemblies are configured with at least one pivotpoints preferably in conjunction with an elastomeric or otherwiseelastically deformable component (such as some form of spring), so asnot to impede the motion (translation), in any plane, of naturalstructures which bear physiologic loads, and their deployment in anorientation in approximately the line of principal compressive stress,i.e., placement at approximately the center of rotation vis à vis ahuman vertebral motion segment which includes two adjacent vertebralbodies and the intervening intervertebral disc.

The assemblies may be inserted axially within the spine, followingeither partial or complete nucleectomy and through a cannula that isdocked against the sacrum, into a surgically de-nucleated disc space,from said access point across a treatment zone. In one aspect of thedisclosure, prosthetic or augmentation materials are introduced, throughat least one vertebral body or into at least one disc space. Theintroduction of the spinal motion preservation assembly of the presentdisclosure is accomplished without the need to surgically create ordeleteriously enlarge an existing hole in the annulus fibrosus of thedisc, and their deployment therapeutically preserves the physiologicalfunction of natural disc structures.

In one aspect of the disclosure, risks associated with implantexpulsion, migration, or subsidence (that are inherently less for thespinal motion preservation assembly of the present disclosure) may beeven further mitigated by retention means, e.g., by external,self-tapping threads configured to distribute stress evenly over a largesurface area, that engage the vertebral body and secure (i.e., anchor)the implant assemblies therein.

The screw threads are typical of “cancellous” type bone threads known inthe art. The threads are typically cut with generally flat faces on theflights of the thread with the most flat of the faces oriented in thedirection of the applied load. In one embodiment, the thread profilegenerally consists of deep flights with an asymmetric thread form, whichprovides the advantage of improved weight bearing and load distribution.Threads are formed on root portions and extend as continuous threadsfrom the trailing end to the leading end of the respective threadedsections. The screw threads include multiple revolutions that are spacedapart along the roots by inter-thread spacings. Installation issimplified by delivery of the two bone anchors via timed delivery ofthreaded components as described in more detail below and thustypically, the proximal component and distal component threads arelike-handed (i.e. the threads turn in the same direction) so that bothscrew threads are right-handed or so that both are left-handed.

In the context of the present disclosure, “dynamic” refers to non-staticdevices with an inherent ability to allow mobility by enabling orfacilitating force or load bearing that assist or substitute forphysiological structures that are otherwise compromised, weakened orabsent. The spinal motion preservation assemblies (MPA) of the presentdisclosure provide dynamic stabilization (DS) across aprogression-of-treatment interventions for treating symptomaticdiscogenic pain, ranging from treatment in patients where littledegeneration or collapse is evident radio-graphically, to those for whomprosthetic nucleus devices or total disc replacements are indicated. Forexample, a prosthetic nucleus (PN) would be indicated in patients with agreater degree of degeneration and loss of disc height but not to thestage where advanced annular break-down is present. A prosthetic nucleuswould go beyond dynamic stabilization by including an aggressivenucleectomy and subsequent filling of the de-nucleated space with anappropriate material. Here, the goal is to restore disc height andmotion. Total disc replacement (TDR) is generally indicated with moreadvanced disease than with a prosthetic nucleus but where some annularfunction remains. Many of the motion preservation assemblies of thepresent disclosure serve as prosthetic disc replacements (PDR) that aremuch less invasive (in terms of deployment by trans-sacral access) thantraditional total disc replacements, and are configured so as toaugment, preserve, restore, and/or manage the physiological functionaccording to the intervention indicated. In general, the axial motionpreservation assemblies of the present disclosure disclosed herein arepreferably configured as devices with an aspect ratio of greater thanone, i.e., the device dimension in the axial vertebral direction isgreater than the device dimension in any orthogonal direction to thataxial direction in close proximity to the physiological instantaneouscenter of axial rotation, and are deployed in an orientation inapproximately the line of principal compressive stress, and placed atapproximately the center of rotation vis à vis a human disc motionsegment.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 identifies the sections of a human spine.

FIGS. 2( a)-(c) illustrate an anterior trans-sacral axial access methodof creating an axial channel in the spine which can be used to preparean axial channel in the spine for use with the present disclosure

FIG. 3 illustrates an implanted motion preservation assembly 300 in aspinal motion segment.

FIG. 4 is an exploded diagram that provides an enlarged view of thecomponents described in FIG. 3.

FIG. 5 is an exploded diagram that provides another view of thecomponents described in FIGS. 3 and 4.

FIG. 6 is a view with a partial quarter round section removed from thedistal bone anchor 340 and the proximal bone anchor 344 to reveal thecomponents within an assembled set of components.

FIG. 7 is a view of the same motion preservation assembly 300 but with aquarter round of the entire assembly removed.

FIG. 8A illustrates the effect of using two different pivot body widths.

FIG. 8B illustrates the effect of changing the depth of the pivot endcup.

FIG. 8C illustrates the use of a cavity bevel 636.

FIG. 9 illustrates the advantage of a dual pivot over a single pivot.

FIG. 10 illustrates a non-symmetric cavity (raceway 620) which serves asthe constrained area for the pivot 608 to move within to allow for agreater amount of additional allowed translation in one direction ascompared with another direction.

FIG. 11 illustrates an asymmetric pivot.

FIG. 12 is high level flow chart that is useful to introduce the overallsequence of events for delivery of a spinal motion preservation assemblyof the type illustrated in FIGS. 3-7

FIG. 13 is a flow chart containing one set of steps that could be usedto prepare an axial channel via an anterior trans-sacral axial approachfor use with distal and proximal anchors having the same major diameter.

FIG. 14 is a flow chart with one set of steps to deliver a spinal motionpreservation assembly of the type illustrated in FIGS. 3-7.

FIG. 15 is a perspective view of exchange cannula 704.

FIG. 16 is a perspective view of the dual anchor driver 2000 with aquarter round removed to better show the components.

FIG. 17 is a perspective view of retention rod 2004 and shows theretention rod knob 2032 and the threaded distal tip 2036.

FIG. 18 is a perspective view of an insertion tip 2008 with a quarterround removed.

FIG. 19 is a cross section of the insertion tip 2008 shown in FIG. 18.

FIG. 20 is a cross section of a driver shaft 2012 and a female hexagonaldistal end 2052.

FIG. 21 is a cross section view of retainer lock 2020.

FIG. 22 is a cross section of lock stop 2024.

FIG. 23 is a side view of distraction sleeve 2016.

FIG. 24 is a perspective view with a quarter round section removed of adistraction sleeve 2016.

FIG. 25 is a perspective view of distraction driver 2100 with a quarterround removed.

FIG. 26 is a side perspective view of a membrane tip 2148 with amembrane 2152.

FIG. 27 is a perspective view with a quarter round removed of themembrane tip 2148.

FIG. 28 is a cross section of the membrane tip 2148.

FIG. 29 is a perspective view with quarter round removed of adistraction driver tip 2108.

FIG. 30 is a cross section of the distraction driver tip 2108.

FIG. 31 is a perspective view with quarter round removed of a distal cupdriver 2200.

FIG. 32 is a perspective view of a mandrel shaft 2224 component of adistal cup driver 2200.

FIG. 33 is an enlarge enlarged perspective view of the expanding mandrel2204 component of a distal cup driver 2200.

FIG. 34 is a perspective view of a plug shaft 2228 component of a distalcup driver 2200.

FIG. 35 is an enlarged portion of a perspective view of a portion of aplug shaft 2228 component of a distal cup driver 2200.

FIG. 36 is an enlarged portion of a perspective view of the proximal endof a distal cup driver 2200.

FIG. 37 is a perspective view of a support member driver 22601

FIG. 38 is a cross section of a support member driver 2260.

FIG. 39 is a perspective view with a quarter round removed of a dual usedriver 2300.

FIG. 40 is a cross section of the dual use driver 2300.

FIG. 41 shows an enlarged perspective view of insertion tip 2304 withthe distal end in the foreground, and also shows one of the two pinengagement holes 2328 used for engagement with a retaining pin 2312.

FIG. 42 shows a perspective view of a retention rod 2316 including aknob 2340 and threaded tip 2344.

FIG. 43 is a perspective view of the proximal anchor stabilizer 2380.

FIG. 44 is a side view of the proximal anchor stabilizer 2380.

FIG. 45 is an enlarged perspective view of a stabilizer tip 2392.

FIG. 46 illustrates a cross section of a support member 900 having amachined spring 916.

FIG. 47 is a side view of the modified distal cup 2372.

FIG. 48 is a cross section of the modified distal cup 2372.

FIG. 49 is a perspective view of the modified distal cup 2372 with theproximal end of the distal cup in the foreground.

FIG. 50 is a perspective view of the distal cup 2372 with the distal endin the foreground.

FIG. 51 shows a perspective view of an alternative distal cup 2400 witha partial quarter round removed.

FIG. 52 shows a perspective view of an alternative distal cup 2400 witha partial quarter round removed after compression has caused elastomericcomponent 2408 to expand to fill the cavity in the alternative distalcup.

FIG. 53 shows a perspective view with a quarter round removed of apreformed membrane 2450 with a one inch diameter (as measured inside thepreformed membrane before adding silicone material).

FIG. 54 shows a cross section of the preformed membrane 2450 of FIG. 53.

FIG. 55 shows a cross section of a ¾ inch preformed membrane 2460 thatcould be delivered by the same delivery device and membrane tip but maybe preferred by a surgeon working with a disc that has a smallerdiameter.

FIG. 56 shows a detail applicable to both FIGS. 54 and 55, the membranechannel engagement section 2454 of one side of a membrane.

FIG. 57 is a perspective view of an alternative flat membrane 2470.

FIG. 58 is a cross section of the flat membrane 2470 of FIG. 57.

FIG. 59 shows a cross section of a spine with an implanted prostheticnucleus 2504.

FIG. 60 shows a membrane tip 2520 with a preformed membrane 2524.

FIG. 61 is a perspective view of a two level spinal motion preservationassembly 3500 with the membranes hidden to allow a better view of thecomponents.

FIG. 62 is an exploded view of the various components beyond the threebone anchors and two membranes (membranes not shown) that are in the twolevel spinal motion preservation assembly 3500.

FIG. 63 is a cross section of a spinal motion preservation assembly 3700that uses a single pivot.

FIG. 64 introduced concepts relevant to having multiple bearingsurfaces.

FIG. 65 is an exploded diagram of the components for a spinal motionpreservation assembly with a pair of O-rings to allow for elastomericdeformation and thus motion along the Z axis.

FIG. 66 is high level flow chart that is useful to highlight thedifferences in the overall sequence of events for delivery of a spinalmotion preservation assembly as shown in FIG. 65 as compared with thetype illustrated in FIGS. 3-7.

DETAILED DESCRIPTION

Because of the many advantages associated with a minimally invasive, lowtrauma trans-sacral axial approach, the present disclosure contemplatesthe use of the trans-sacral axial access to the lumbo-sacral spine. Thetrans-sacral axial approach (described and disclosed in commonlyassigned U.S. Pat. Nos. 6,558,386; 6,558,390; 6,575,979; 6,921,403;7,014,633, and 7,087,058) has a number of advantages over other routesfor delivery of therapeutic devices to motion segments but there arelogistical challenges to the delivery and installation of advancedspinal assemblies via an axial access channel. The process of addressingthese challenges impacts certain aspects of the implanted device andobviously impacts the design of the insertion tools.

Trans-Sacral Axial Access

The trans-sacral axial access method illustrated in FIG. 2, eliminatesthe need for muscular dissection and other invasive steps associatedwith traditional spinal surgery while allowing for the design anddeployment of new and improved instruments and therapeuticinterventions, including stabilization, motion preservation, andfixation devices/fusion systems across a progression-of-treatment inintervention.

FIG. 2 provides an introductory overview of the process with FIGS. 2( a)and 2(b) showing the process of “walking” a blunt tip stylet 204 up theanterior face of the sacrum 116 to the desired position on the sacrum116 while monitoring one or more fluoroscopes (not shown). This processmoves the rectum 208 out of the way so that a straight path isestablished for the subsequent steps. FIG. 2( c) illustrates arepresentative trans-sacral axial channel 212 established through thesacrum 116, the L5/sacrum intervertebral space, and into the L5 vertebra216. If therapy is being provided to the L4/L5 motion segment then thechannel would continue through the L5 vertebra 216 through the L4/L5intervertebral space, and into the L4 vertebra 220.

The discussion of FIG. 2 provides context for the present disclosure.Previous applications (some now issued as United States patents)assigned to TranS1, Inc. have included a description of an alternativeaccess method that is a posterior trans-sacral axial spinal approachrather than an anterior trans-sacral axial spinal approach. (See e.g.U.S. Pat. No. 6,558,386 for Axial Spinal Implant and Method andApparatus for Implanting an Axial Spinal Implant Within the Vertebrae ofthe Spine as this patent describes the anterior trans-sacral axialapproach illustrated in FIG. 2 and is incorporated by reference in itsentirety.) Many of the teachings of the present disclosure, and inparticular devices as shown in FIGS. 3-7, can be utilized with atrans-sacral axial access method.

A brief overview of this method of accessing the spinal region toreceive therapy is useful to provide context for the present disclosure.As shown in FIG. 2A, a pre-sacral approach through percutaneous anteriortrack towards sacral target, through which trans-sacral axial bore willbe made and channel extended distally for subsequent advancement of aspinal motion preservation assembly. An anterior, pre-sacral,percutaneous tract extends through the pre-sacral space and ends on theanterior face of the sacrum. The pre-sacral, percutaneous tract ispreferably used to introduce instrumentation to access and prepare(e.g., by drilling a bore in the distal/cephalad direction through oneor more lumbar vertebral bodies and intervening discs). “Percutaneous”in this context simply means through the skin from a paracoccygealaccess point on the patient and to the posterior or anterior targetpoint, as in transcutaneous or transdermal, without implying anyparticular procedure from other medical arts. However, percutaneous isdistinct from a surgical access, and the percutaneous opening in theskin is preferably minimized so that it is less than 4 cm across,preferably less than 2 cm. The percutaneous pathway is generally axiallyaligned with the bore extending from the respective anterior orposterior target point through at least one sacral vertebral body andone or more lumbar vertebral body bodies in the cephalad direction asvisualized by radiographic or fluoroscopic equipment. Additional detailsregarding the process of preparing an axial access channel may be foundin co-pending and commonly assigned U.S. patent application Ser. Nos.10/972,065, 10/971,779; 10/971,781; 10/971,731; 10/972,077; 10/971,765;10/971,775; 10/972,299; and 10/971,780, all of which were filed on Oct.22, 2004, and commonly assigned U.S. Provisional Patent Application60/706,704, filed Aug. 9, 2005, and all of which are incorporated byreference herein in their entirety.

First Example

The present disclosure will now be described more fully hereinafter withreference to accompanying drawings in order to disclose selectedillustrative implementations of the present disclosure. The teachings ofthe present disclosure may, however, be embodied in many different formsand should not be construed as limited to the particular implementationsset forth herein; rather these implementations are provided so that thedisclosure can be thorough and complete, and as part of the effort toconvey the scope of the disclosure to those skilled in the art. Likenumbers refer to like elements throughout.

In order to avoid the imprecision that can sometimes be introduced intoa patent application while discussing many different alternativeconfigurations at once, FIGS. 3-7 start with one very specificembodiment of the present disclosure. In order to provide an overview ofthe components and their placement with respect to a spinal motionsegment, the explanation will start with an overview of an implanteddevice. Subsequent drawings will provide detail on the delivery andassembly of the device.

FIG. 3 illustrates an implanted motion preservation assembly 300. FIG. 4provides an enlarged view of the implanted motion preservation assembly300 in order to allow more room for reference numbers. FIG. 5 providesan exploded diagram that provides another view of the componentsdescribed in FIGS. 3 and 4. In order to avoid undue clutter from havingtoo many reference numbers and lead lines on a particular drawing, somecomponents will be introduced via one drawing and not explicitlyidentified in every subsequent drawing that contains that component.

This motion preservation assembly 300 is implanted into a distalvertebral body 304 and a proximal vertebral body 308. As shown in FIG. 3by way of example, the distal vertebral body is the L5 vertebra 216 andthe proximal vertebral body is the sacrum 116. The installed motionpreservation assembly 300 extends across an intervertebral disc space312. The motion preservation assembly 300 would be placed in apreviously prepared axial channel 212. The trans-sacral axial approachleft intact the axial walls of the annulus fibrosus 254. Collectively,the distal vertebral body 304, the proximal vertebral body 308, and theintervertebral disc space 312 form a motion segment 316 (as the proximalbody in FIG. 3 is the sacrum, only the upper portion of the sacrum isshown within bracketed area 316). The drawings of the vertebral bodiesin this Figure are not intended to convey anatomical details of thespinal components but to illustrate the placement of the assembledmotion preservation assembly 300. In a like manner, other Figures inthis disclosure are used to disclose specific concepts rather than toconvey details of human anatomy. While the example pair of adjacentvertebral bodies used in FIGS. 3 and 4 are L5 and the sacrum (or to bemore specific S1), other motion segments can receive a spinal motionpreservation assembly using a trans-sacral axial approach. It isbelieved that the second most common location for installation of aspinal motion preservation assembly via an axial trans-sacral approachwill be between the L4 and L5 vertebrae 220 and 216 (See FIG. 2), butother motion segments may benefit from such devices.

The major components of the motion preservation assembly 300 include thedistal component 340 (anchored in the superior, or distal vertebralbody, herein also sometimes referred to as distal bone anchor), proximalcomponent 344 (anchored in the inferior, or proximal vertebral body,herein also sometimes referred to as proximal bone anchor), prostheticnucleus 348 (generally including outer membrane 460), and a supportmember 352. An optional seal ring 396 (shown most clearly in FIG. 5)will be discussed in connection with the assembly of the device.

The distal bone anchor 340 shown in FIG. 4 has a set of external threads356. Advantageously, the set of external threads 356 can include a chipbreaker section 360 (FIG. 5) at the distal end of the distal bone anchorto facilitate the starting of cutting a thread path into the distalvertebral body 304. A chip breaker is a discontinuity in the thread thatallows chips to break off as the thread path is cut. The axial channel212 is created into the distal vertebral body 304, with the diameter ofthe axial channel 212 at the distal vertebral body 304 approximatelyequal, or slightly less than, the minor diameter of the set of externalthreads 356.

The distal bone anchor 340 has a cavity 364 (best seen in FIG. 5)running from the distal face 366 of the distal bone anchor 340 to theproximal face 370 (FIG. 5) of the distal bone anchor 340. In thiscontext, a face is the three dimensional surface of the part as viewedfrom that side, akin to the six three dimensional faces of die from apair of dice. The cavity 364 is not of uniform cross section and servesseveral purposes. The distal end of the cavity 364 extends to the distalface 366 of the distal bone anchor 340 such that the cavity can be usedto allow the distal bone anchor 340 to be deployed over a guide wire(not shown). The cavity 364 includes an internal threaded section 368which can be engaged by a retention rod (See 2004 in FIGS. 16 and 17) asdescribed below. As shown in FIG. 4, the internal threaded section 368is engaged with external threads 376 on a distal cup 372. The distalbone anchor 340 is adapted to be driven by a polygonal driver receivedin the proximal end of the cavity 364 in the distal bone anchor 340. Inthis implementation, the female hex is little more than a set of hexridges 374.

The cavity 364 in the distal bone anchor 340 shown in FIG. 4 issubstantially filled with the distal portion of the distal cup 372. Thedistal cup 372 extends beyond the proximal face 370 of the distal boneanchor 340 into the intervertebral disc space 312. If the proximal face370 of the distal bone anchor 340 is positioned to be roughly coplanarwith the proximal face of the distal vertebral body 304, the distal endof the support member 352 is in that portion of the bore that is in thevicinity of what used to be the endplate for that piece of the distalvertebral body 304. Thus, the spinal motion preservation assembly has apivot point close to the pivot point of a healthy motion segment.

The distal cup 372 in turn has a cavity 380 (best seen in FIG. 5) whichserves as a bearing surface for the distal end 384 of the support member352.

The support member 352 shown in FIGS. 3-5 has the distal end 384(referenced above) and a proximal end 388 which in this embodiment areconfigured as substantially spherical components integral with the body392. While the distal end 384 in FIG. 5 is interchangeable with theproximal end 388, other implementations may have differences between thetwo ends.

The proximal bone anchor 344 has a set of external threads 404. Theproximal bone anchor 344 has a cavity 412 (best seen in FIG. 5) thatruns from the proximal face 408 (best seen in FIG. 5) of the proximalbone anchor 344 to the distal face 414 (best seen in FIG. 4) of theproximal bone anchor 344. The cavity 412 is not uniform in crosssection. A portion of the cavity 412 has a set of internal threads 416.The pitch of the set of internal thread 416 will be relatively fine(perhaps 16 threads per inch up to 64 threads per inch). For example athread pitch of 32 threads per inch may be acceptable for some uses.

In the implementation shown in FIGS. 3-5, the proximal bone anchorcavity 412 contains a proximal cup 420 that has a set of externalthreads 424 that engage with the set of internal threads 416 to allowtorque from a driver imparted to a driver engagement section 428 (bestseen in FIG. 5) to rotate the proximal cup 420 relative to the proximalbone anchor 344 to axially advance the proximal cup 420. Axial threadgrooves 426 in the external threads 424 make the external threads 424less susceptible from problems arising from small amounts of prostheticnucleus material (such as silicone) which may get into the internalthreads 416 in the proximal bone anchor 344.

One can appreciate that axially advancing the proximal cup 420 willcause the proximal cup 420 to contact the support member 352 and causethe support member 352 to in turn contact the distal cup 372. Afterthese components are in contact, further axial advancement of theproximal cup 420 will cause the axial movement of the distal bone anchor340 as it contains distal cup 372 (and the distal vertebral body 304engaged with the distal bone anchor 340). This axial movement of thedistal bone anchor 340 will be relative to the proximal bone anchor 344(and the proximal vertebral body 308 engaged with the proximal boneanchor 344). This movement of one vertebral body away from anothervertebral body results in distraction, that is an increase of theintervertebral disc space between the two vertebral bodies.

Note that the distraction that can be achieved by rotation of theproximal cup 420 is preferably used as an adjustment to alter thedistribution of loading between the support member and the prostheticnucleus component of the spinal motion preservation assembly. Theprimary means for achieving distraction is by use of the distractiondriver 2100 as will be described below.

Returning to FIGS. 3-5, the driver engagement section 428 could beconfigured among any one of many types of ways to impart torque with adriver. A female hex socket is a suitable choice. The proximal cup 420includes a threaded cavity 432 (best seen in FIG. 4) which can beengaged with a driver or extraction tool. The proximal cup 420 includesa distal cavity 436 that serves as a bearing surface for the proximalend 388 of the support member 352.

The cavity 412 in the proximal bone anchor 344 may also include a jamnut 440 with a distal end 442 and a proximal end 450. The jam nut 440has a set of external threads 444 adapted to engage with the set ofinternal threads 416. As with the external threads 424 on the proximalcup 420, the external threads 444 on the jam nut 440 have a set of axialthread grooves 446 to make the external threads 444 less susceptible toproblems from prosthetic nucleus material (such as silicone) thatbecomes present in the internal threads 416 of proximal bone anchor 344as the thread grooves 446 allow the jam nut 440 to be axially advancedthrough the threaded engagement despite the presence of some amount ofsilicone. The jam nut 440 also has a driver engagement section 448 thatis adapted to receive torque imparted by a corresponding driver such asa male hex driver. The torque input can cause the jam nut 440 to axially(distally) advance until it makes contact with the proximal cup 420. Thejam nut 440 shown in FIG. 3 also includes a threaded cavity 452 (bestseen in FIG. 4) which can be used by a driver or extraction tool.

The proximal bone anchor 344 also includes a set of slots 456 on theproximal end of the proximal bone anchor 344. Note that the externalthreads 404 continue into the portion of the proximal bone anchor 344that has the set of slots 456. (Note the cross section seen in FIGS. 3and 4 passes through a slot 456 on the right side of the proximal anchorwhich makes it look like the external threads end prematurely on thatside). As described in more detail below, these slots 456 may be engagedby a corresponding set of fingers (See 2384 in FIG. 45) on a tool sothat the axial position of the proximal bone anchor may 344 bemaintained by preventing rotation of the proximal bone anchor 344 whiletorque is applied to either the proximal cup 420 or a jam nut 440.

The prosthetic nucleus 348 includes an outer membrane 460 and prostheticnucleus material 464. In one implementation, as outer membrane 460 isfilled with prosthetic nucleus material 464, the outer membrane 460expands to conformably contact the inferior endplate of the distalvertebral body 304, the superior endplate of the proximal vertebral body308, and the inner wall of the annulus fibrosus 254 which collectivelydefine the boundaries of an intervertebral disc space.

As FIG. 5 shows the various components before insertion into the bodywith the exception of the outer membrane 460 which is delivered mountedon a special delivery device described below in connection with commentsabout the process to delivery the components of the spinal motionpreservation assembly.

FIGS. 6 and 7 provide additional views of motion preservation assembly300 previously discussed in connection with FIGS. 3-5. FIG. 6 provides aview with a partial quarter round section removed from the distal boneanchor 340 and the proximal bone anchor 344 to reveal the componentswithin an assembled set of components. FIG. 7 is a view of the samemotion preservation assembly 300 but with a quarter round of the entireassembly removed.

FIG. 6 shows the internal threaded section 368 of the distal bone anchor340 and the external threads 376 on the distal cup 372. The seal ring396 is visible at the end of the internal threads 416 of the proximalbone anchor 344. In FIG. 6, unused threads 418 of the set of internalthreads 416 are visible as the proximal cup 420 could be axiallyadvanced relative to the proximal bone anchor 344. A cross section ofone of the shallow hex ridges 374 is visible in FIG. 6. FIG. 7 shows theupper portion of a jam nut 440 within but not contacting the driverengagement section 428 on the proximal cup 420.

Alignment marks 472 are visible in FIG. 6. The quarter round crosssection in FIGS. 6 and 7 and the cross section in FIGS. 3 and 4 aretaken through a set of cuts in the outer thread in the distal anchor 340and proximal bone anchor 344. These alignment marks 472 are one way ofmarking the anchors so that they can be loaded on a driver for timeddelivery.

A timed delivery of the two bone anchors 340 and 344 allows for controlover the rotational position of the two sets of threads. The purpose ofthis controlled delivery is to avoid cross threading. More specifically,when electing to use the same thread pitch for the external threads 356on the distal bone anchor 340 and on the external threads 404 on theproximal bone anchor 344, the distal bone anchor 340 can be made with aslarge a cross section (rod diameter) as the proximal bone anchor 344.Having a large cross section is desirable as it makes it easier todesign a distal bone anchor with adequate strength and maximumengagement between the threads and the bone of the distal vertebral body304.

If the minor diameters and the major diameters of the threaded portionsof distal bone anchor 340 and the proximal bone anchor 344 are the same,then the bores through the proximal vertebral body 308 and the distalvertebral body 304 created during the process of creating the axialchannel 212 could be the same size. As the distal bone anchor 340 ismoved towards the bore in the distal vertebral body 304, the distal boneanchor 340 is first axially advanced by rotating it through the bore inthe proximal vertebral body 308. As the distal bone anchor 340 isrotatably advanced through the proximal vertebral body 308, the externalthreads 356 cut a helical thread path into the bone around the bore inthe proximal vertebral body 308 as the bore is approximately the size ofthe minor diameter of the external threads 356 (the bore may be slightlysmaller than the minor diameter) and the major diameter of the externalthreads 356 extends beyond the bore into the bone. Without timing orkeyed delivery, the subsequent axial advancement of the proximal boneanchor 344 would tend to cut a new helix into the bone around the borein the proximal vertebral body 308. This second helix would meet addedresistance as the bone has just received a newly cut thread helix, andthe strength of the connection between the external threads 404 on theproximal bone anchor 344 is compromised by previously cut and now unusedthread path through the bone. In contrast, timed delivery allows theleading edge of the helical thread on the exterior of the proximal boneanchor 344 to enter into the helical thread path left by the externalthreads 356 on the distal bone anchor 340. An alternative to timeddelivery is to size the major diameter of the external threads on thedistal bone anchor to be less than the diameter of the bore in theproximal vertebral body and then prepare a bore in the distal vertebralbody that is approximately the size of the minor diameter of theexternal threads on the distal bone anchor. A variation of thisalternative is to have the major diameter of the external threads on thedistal bone anchor to be just slightly larger than the bore in theproximal vertebral body so that the distal bone anchor may be rotated toaxially advance through the proximal vertebral body, but the resultinghelical thread path is not very deep and does not prevent the subsequentproximal bone anchor with a larger major diameter from cutting a deeperhelical thread path to firmly anchor the proximal anchor. The minordiameter of the proximal bone anchor may slightly exceed the bore in theproximal vertebral body.

Returning now to FIG. 4, it is preferred that the bulk prostheticnucleus materials (PNM) (element 464 in FIG. 4) include elastomericsolids and/or viscoelastic gels, i.e., materials whose viscoelasticproperties (e.g., rheology) alone or in conjunction with thebiomechanical properties of outer expandable membrane 460, enable themto perform in a functional manner which is substantially equivalent tothe physiologic disc nucleus. Preferred prosthetic nucleus materials andsystems may use biomedical grade silicone elastomer e.g., siliconerubber, such as that obtained from Nusil Silicone Technology located inCarpeneria, Calif. or hydrogels or blends thereof (e.g.,hydrogel/hydrogel, or hydrogel/elastomer). Cross-linked hyaluronic acid,such as is available from Fidia Corporation in Italy, is an example of asuitable material, however, many natural and man-made hydrogels orblends thereof may be configured to achieve similar properties withoutinflammatory response, such as those disclosed and described inco-pending and commonly assigned United States Patent Applicationsreferenced above, and in detail in particular in U.S. Provisional PatentApplications 60/599,989 filed Aug. 9, 2004, and 60/558,069 filed Mar.31, 2004, each of which are incorporated in their entirety into thisdisclosure by reference. Priority is also claimed to co-pending andcommonly assigned U.S. patent application Ser. No. 11/199,541 filed Aug.8, 2005 for “Prosthetic Nucleus Apparatus and Methods” and theprovisional 60/599,989 filed Aug. 9, 2004 claimed as a priority documentin the '541 application. Both of these applications are incorporated intheir entirety into this disclosure by reference.

While other prosthetic nucleus membranes will be described asalternatives in a subsequent portion of this document, one combinationthat is of interest is a silicon membrane filled with silicon as thesilicon used to fill the membrane effectively becomes functionallyindistinguishable post cure with the silicone membrane.

Degrees of Freedom and Limitations

As FIGS. 3 through 7 are static and the desired attributes of a motionpreservation assembly are dynamic stabilization, it is appropriate todwell on how the motion segment can move with an implanted spinal motionpreservation assembly.

Looking at FIG. 4, if one looks at pivot point 480 located just abovethe threaded cavity 432 in the proximal cup, one can start to count theways that the distal bone anchor 340 and corresponding distal vertebralbody 304 can move relative to the pivot point 480. The first type ofmovement is axial rotation (clockwise or counterclockwise) around the Zaxis. Nothing in this particular spinal motion preservation assemblyplaces a limit on the amount of clockwise or counterclockwise motions.As discussed above, a system that allows about two degrees of rotationabout this Z axis would support the normal range of motion on this axis.

As the normal full range of motion typically allows for about 12 degreesof flexion, about 8 degrees of extension, about 9 degrees of left orright lateral bend, in order to avoid constraining the normal range ofmotion, an installed spinal motion preservation assembly would need toallow at least these amounts of rotation in the relevant planes. Theprecise range and degree of motion for a motion segment varies along themotion segments of the spine. For example the range and degree of motionin the L4-L5 motion segment will not be exactly the same as for theL5-S1 motion segment.

The device shown in FIG. 4 is radially symmetric around the Z axis so itdoes not need to be positioned in a particular orientation in order toprovide the maximum capacity for rotation in a particular direction (forexample flexion versus extension or lateral bending). The rotation ofthe implanted distal bone anchor with respect to the proximal boneanchor can be achieved through a combination of the action of theproximal end 388 of support member 352 moving with respect to theproximal cup 420 and the action of the distal end 384 of support member352 moving with respect to the distal cup 372.

Support member 352 discussed above, has a pair of pivots. One can changethe maximum pivot angle of a pivot with respect to the corresponding cupby varying two parameters, the depth of the cavity in the cup and thewidth of the pivot body (compare with support member body 392 in FIG. 5)with respect to the pivot cavity. FIG. 8A and FIG. 8B illustrate theseconcepts. FIG. 8A illustrates two different pivot body widths. If apivot body was changed from width 608 to width 612, then the wider pivotbody 612 would hit the pivot end cup 604 after a smaller amount ofrotation than if the pivot body remained with width 608.

FIG. 8B uses the pivot body of FIG. 8A but places it in a pivot end cup616 that is not as deep so that the pivot can rotate with respect to thepivot end cup more in FIG. 8B than in FIG. 8A. Comparing FIG. 8C to FIG.8B shows another way to increase the range of motion for a pivot. Pivotend cup 632 is much like pivot end cup 616, except that pivot end cup632 has a cavity bevel 636. In some cases the use of a cavity bevel willallow a pivot the ability to move further before coming in contact withthe pivot end cup. The spinal motion preservation assembly shown inFIGS. 3-7 includes a cavity bevel 410 in the proximal bone anchor 344(as best seen in FIGS. 4 and 6). As the proximal cup 420 is recessedrelative to the distal face of the proximal bone anchor 344, it isappropriate to place the bevel on the anchor rather than the cup. Theparticular distal pivot cup shown in FIG. 5 has about a 45 degree bevel406.

The use of two pivots allows for translation of one implanted boneanchor relative to the other in the X axis (anterior/posterior), Y axis(lateral) or a combination of the two. In order to appreciate theadvantage of using two pivots, it is useful to look at the movement whenthere is one pivot. In FIG. 9( a), a single pivot 650 is engaged with abearing surface in proximal bone anchor 654 and fixed to distal boneanchor 658. When the pivot 650 rotates in the X-Z plane away from the Zaxis as shown in FIG. 9( b), the distal bone anchor 658 moves inrotation with respect to the proximal bone anchor 654 to change therelative orientation of the one bone anchor to the other.

In contrast, in FIG. 9( c), the pivot 660 is a dual pivot and thus canpivot with respect to proximal bone anchor 654 or distal bone anchor668. Now the distal bone anchor 668 can move in substantially puretranslation along the X axis with respect to the proximal bone anchor654. The relative orientations of the bone anchors are preserved as themovement did not impose a rotation on the distal bone anchor 668. Theterm substantially pure translation was used as the elevation of thedistal bone anchor 668 changed slightly during the movement from thestate shown in FIG. 9( c) to the state shown in FIG. 9( d) which wouldnot happen in pure X axis translation. While this example showedtranslation in the X axis, the same type of movement as shown in FIG. 9would happen in the Y axis or in a mix of X and Y components unless thepivots were restrained in some way.

While the example of the spinal motion preservation assembly shown inFIGS. 3-7, provides significant capacity for X or Y translation,additional translation could be allowed by expanding the cavities in thecups to exceed the maximum diameter of the pivot end sphere by 5millimeters. This added cavity size will provide some additionalcapacity for translation.

The example shown in FIGS. 3-7 uses radially symmetric pivot ends andcup cavities. Using a non-symmetric cavity in the cup could allow for agreater amount of additional allowed translation in one direction ascompared with another direction. FIG. 10 shows an example of thisconcept. Looking down on a pivot 608, the wall of the pivot body and themaximum dimension of the spherical head of the pivot are both shown.Also shown is raceway 620 which serves as the constrained area for thepivot 608 to move within. Note that additional translation in the Xdirection is extremely limited but a much greater amount of additionaltranslation is possible in the Y (lateral) direction. When using such anasymmetric raceway, the insertion technique would need to control theorientation of the bone anchor so that the elongated direction of theraceway was aligned in the proper direction. One method is to insert thecomponents in a particular orientation with respect to the driver. Thedriver would have a marker on it so that the driver marker could bemonitored to ensure placement of the components in proper orientationwith the anterior/posterior and lateral axes. Orientation of cups withininserted bone anchors could be controlled by having a key and slotengagement between the cup and bore within the anchor.

Another way to allow more bending in one direction than in the other isto have an asymmetric pivot. FIG. 11 illustrates an asymmetric pivot 624(only one end of the pivot is shown) in a cup 628. This asymmetric pivotprovides more limitation on the movement in the X direction(anterior/posterior) than in the Y direction (lateral left/lateralright). When using an asymmetric component, it is important that theinstallation procedure place the asymmetric component in the properstarting position. Note, that although the device supports unlimitedrotation along the Z axis, the positioning of the asymmetric pivot willremain relatively constant as the range of axial rotation for anindividual motion segment is only approximately two degrees clockwiseand approximately two degrees counterclockwise.

In order to provide the 6^(th) degree of freedom, translation in the Zaxis, the motion preservation assembly would need to allow the distalbone anchor to move in the Z axis relative to the proximal bone anchor.(A careful observer will note that the translation in the X or Y axisachieved through the use of dual pivot points will incidentally providefor a change in the Z axis but a force purely along the Z axis would notcause this sort of translation.) Adding a degree of freedom for Ztranslation can be achieved in theory by a device that elongates intension. However, tensile loads that impart a distraction on a motionsegment are rare. The most common being the therapeutic application oftraction to extend the spine.

Thus, the more useful capacity for translation in the z-axis is theability of the spinal motion preservation assembly to compress. Thereare many motion segments in a spine so it is not surprising that theamount of compression for an individual motion segment is not a largedistance. A healthy motion segment in a spine may be capable of about0.9 millimeters of compression with approximately 0.5 millimeters ofcompression attributed to the nucleus and approximately 0.2 millimetersof compression attributed to each vertebral body as the endplates oneither side of the nucleus move relative to their respective vertebralbodies. Thus, under an appropriate compressive load, the distance from amidpoint in one vertebral body to the midpoint of an adjacent vertebralbody can be reduced by about 0.9 millimeters. As with most statementsabout motion segments, one caveat is that there is variation inexpectations for motion segments in different parts of the spinal columnand as with many things anatomical, there can be substantial variationbetween people especially when the very young and the very old areconsidered.

It may be advantageous to add to a spinal mobility preservation assemblythe ability to compress along the Z axis. Ideally, the compression wouldbe reversible and repeatable so that the assembly could undergo manycycles of compression and recovery. Thus, the deformation from thecompression would need to be elastic. Elasticity is the property wherebya solid material changes its shape and size under force but recovers itsoriginal configuration when the forces are removed. For many people,elastic deformation brings to mind a rubber ball that can be deformedand then resumes its original shape. Bladders can undergo elasticdeformation (examples include pneumatic tires and the bladders in soccerballs). Elastic deformation can be achieved through use of springsincluding disc springs of various types such as a Belleville disc springor comparable spring.

Addition of an elastically deformable component in the motionpreservation assembly would allow this motion segment to contribute tothe spinal column's ability to compress under a heavy compressive forcesuch as landing on feet after jumping or falling onto the buttocks. Asthe set of motion segments are in a stack, the lack of ability of onemotion segment to elastically compress may be tolerable as it istolerated for the people who have a fused motion segment. Having someability to elastically compress in the Z direction is thought to bedesirable especially for spinal motion preservation assemblies withprosthetic nuclei as these ideally mimic the behavior of a naturalmotion segment.

Elastic deformation of the spinal motion preservation assembly to allowthe endplates of the distal vertebral body and the proximal vertebralbody to move closer together would apply a compressive force on theprosthetic nucleus that assumed a conforming fit shape that contacts theendplates of these vertebral bodies. For at least some choices forprosthetic nucleus, compression of the prosthetic nucleus in the Zdirection causes it to expand radially outward to maintain the overallvolume of the prosthetic nucleus as the material in the prostheticnucleus may be for all practical purposes incompressible (unless therewas a substantial amount of gas in the prosthetic nucleus, the amount ofcompression may be so small as to negligible). As the prosthetic nucleusexpands radially outward, it transfers forces to the various layers ofthe annulus fibrosus, thus mimicking the natural transfer anddissipation of physiologic loads.

Having a spinal motion preservation assembly with the ability to undergoelastic (recoverable) deformation in the Z axis and the ability topromote radial distribution of loads to the annulus fibrosus to mimicnormal physiological load sharing is apt to reduce the risk ofsubsidence or transition syndrome. The discussion of various options forthe introduction of elastically deformable components to allow forelastic deformation along the Z axis of the spinal motion preservationassemblies will be discussed in detail below with the discussion ofalternative implementations of spinal motion preservation assemblies.The use of one or more elastically deformable components may allowcompression of substantially up to the about 0.9 millimeters ofcompression that is possible in some healthy motion segments.

Process to Deliver a Spinal Motion Preservation Assembly

The process to deliver the spinal motion preservation assembly shown inFIGS. 3-7 will be set forth with some level of detail such that theinteraction of the assembly components with the various drivers can beunderstood sufficiently so that one of ordinary skill in the art will beable to recognize and implement the necessary modifications to thedrivers to deliver one of the alternative implementations of the deviceas discussed below, or certain variations of the illustrativeimplementations. A co-pending application for Driver Assembly forSimultaneous Deliver of Spinal Implants U.S. patent application Ser. No.11/259,614 discusses in detail drivers for a different spinal motionpreservation assembly. The '614 application has been incorporated byreference and augments the teachings of this application with respect tothe use of drivers to deliver spinal motion preservation assemblies.

The process to deliver the spinal motion preservation assembly may bedescribed in the context of flow charts and in an examination of driversthat can be used. With respect to the flow chart, it may be useful toview the process at a summary level in one flow chart and then in a moredetailed level in a subsequent flow chart. Even at the more detailedlevel, the flow chart is conveying process steps to one of ordinaryskill in the art and not every movement or sub-step needs to beconveyed.

FIG. 12 is a high level flow chart that is useful to introduce theoverall sequence of events for delivery of a spinal motion preservationassembly of the type illustrated in FIGS. 3-7. This flow chart and themore detailed flow charts contained in FIGS. 13-14 provide details thatare specific to delivery of a specific implementation of the teachingsof this disclosure to a specific motion segment via a specific route.Thus, while there are many possible variations of the ways that spinalmotion preservation assemblies may be implemented, when describing aspecific delivery process for a specific implementation of a spinalmotion preservation assembly, it is appropriate to focus and describe aspecific process. This specificity is thought to be useful inillustrating the interaction between specific portions of the spinalmotion preservation assembly components and the various drivers used inthe delivery process so that one of ordinary skill in the art couldmodify both the components and the drivers as needed to deliver otherspinal motion preservation assemblies incorporating one or moreteachings of the present disclosure. With that understanding of thepurpose of these flow charts, attention is directed to FIG. 12.

1106—Create axial channel 212. This process will be described in moredetail in connection with FIG. 13.

1112—Deliver both bone anchors (340 & 344) to the pair of vertebralbodies and adjust the position of the distal bone anchor 340 relative tothe distal vertebral body 304. As described below, in thisimplementation, the two anchors are delivered by timed delivery on asingle dual anchor driver. As the position of the distal bone anchor 340is adjusted, the dual anchor driver is also engaged with the proximalbone anchor 344.

1118—Adjust the position of just the proximal bone anchor 344 afterdisengaging the dual anchor driver from the distal bone anchor 340.

1124—Distract the intervertebral space 312 between the distal vertebralbody 304 and the proximal vertebral body 308 by forcing an increase indistance between the distal bone anchor 340 and the proximal bone anchor344.

1130—Add prosthetic nucleus material to fill the void in theintervertebral disc space 312 but not in the portion of theintervertebral disc space between the proximal bone anchor 344 and thedistal bone anchor 340 as that space is occupied during filling by aportion of the device used to fill the prosthetic nucleus.

1136—Allow the prosthetic nucleus material to cure so that theprosthetic nucleus can substantially maintain the distraction after thedistraction tool is removed and share some of the load applied to themotion segment.

1142—Deliver the distal cup 372 into the distal bone anchor 340.

1148—Deliver the support member 352 to place the distal end 384 of thesupport member in proximity to the distal cup 372.

1154—Deliver the proximal cup 420 to the proximal bone anchor 344.

1160—Adjust load distribution between that as supported by supportmember (e.g., distributed to bone) and that amount as shared by theprosthetic nucleus (e.g., load transferred to the annulus fibrosus) bymoving the proximal cup 420 relative to the proximal bone anchor 344.

1166—Deliver jam nut to proximal bone anchor 344 to secure the positionof the proximal cup 420.

1172—Close axial channel.

Details on the Creation of an Axial Channel

After that general introduction to the process, FIG. 13 provides a moredetailed description of one set of steps that could be used to preparean axial channel via an anterior trans-sacral axial approach for usewith distal and proximal anchors having the same major diameter. Notethat although FIG. 13 describes a process to provide an access channelfor the delivery of a spinal motion preserving assembly to the L5/S1motion segment, the use of spinal motion preservation assemblies is notlimited to solely that motion segment. As much of the process forpreparing the access channel is the same or similar to processesdescribed in previous applications for this assignee, the steps areassumed to be relatively self-explanatory but are provided here as anoutline that would be meaningful to one of ordinary skill in the art. Asnoted below, spinal motion preservation assemblies could be implementedwith anchors of decreasing major diameters or with a single anchor.Those of ordinary skill in the art could modify the details provided inFIG. 13 and the related text to modify the axis channel preparationprocess accordingly.

1206—Place patient on table in a prone position.

1212—Make longitudinal incision just below and lateral to the coccyxusing a scalpel, incision length of approximately 2 cm.

1218—Insert guide pin introducer with stylet under fluoroscopy into thepresacral space.

1224—Check lateral and anterior/posterior fluoroscopes to verifylocation of guide pin introducer tip. Fluoroscopes will be consulted asneeded for the remainder of procedure to continually verify instrumentposition and trajectory when necessary using lateral andanterior/posterior fluoroscope visualization.

1230—Advance guide pin introducer until it reaches desired entry pointon sacral face. As noted above, the sacrum in an adult is a fused set ofvertebrae given individual names S1 to S5. S1 is the most cephalad ofthese vertebrae.

1236—Remove stylet and replace with guide pin with handle.

1242—Determine proper trajectory and when aligned, tap guide pin intosacrum with slap hammer until guide pin crosses L5-S1 intervertebraldisc space and secures itself in L5 vertebral body.

1248—Remove guide pin handle and attach guide pin extension to guidepin.

1254—Remove guide pin introducer making sure that the guide pin remainsin place.

1260—Pass 6 mm dilator over guide pin and begin driving dilator into thesacrum using the slap hammer over the guide pin.

1266—Continue driving 6 mm dilator into sacrum until the tip reaches theendplate of the sacrum just below the L5-S1 intervertebral disc space.

1272—Remove 6 mm dilator making sure guide pin remains in position andreplace with 8 mm dilator.

1278—Drive 8 mm dilator into sacrum using slap hammer until tip reachesthe endplate of the sacrum just below the L5-S1 intervertebral discspace.

1284—Remove 8 mm dilator making sure guide pin remains in position andreplace with 10 mm dilator with sheath.

1290—Drive 10 mm dilator with sheath into sacrum using slap hammer untiltip reaches the endplate of the sacrum just below the L5-S1intervertebral disc space.

1296—Remove 10 mm dilator body leaving sheath in place and verifyingthat the guide pin remains in position as well.

1302—Insert 9 mm cannulated drill over the guide pin and into the 10 mmdilator sheath.

1308—Twist drill through the sacrum and into L5-S1 disc space and thenremove drill leaving guide pin in position.

1314—Insert 10 mm dilator body into its sheath and remove sheath fromsacrum leaving guide pin in position.

1320—Pass 12 mm dilator over guide pin and drive into sacrum using theslap hammer until tip is in L5-S1 disc space.

1326—Remove 12 mm dilator making sure guide pin remains in position andreplace with 13 mm dilator with sheath.

1332—Drive 13 mm dilator with sheath into sacrum using slap hammer untiltip reaches into L5-S1 disc space.

1338—Remove 13 mm dilator body leaving sheath in place and then removeguide pin with extension.

1344—Insert 12.5 mm drill into 13 mm sheath and drill through sacruminto L5-S1 disc space.

1350—Remove drill and insert cutter body bushing into 13 mm sheath. Thebushing takes up room in the sheath so that the cutter and the 9 mmdrill both travel in a constrained space.

1356—Perform nucleectomy on L5-S1 disc space using radial cutters andtissue extractors being careful to maintain cartilage and endplates.

1362—Optional step: use small radial cutter to countersink endplate ofsacrum to remove any chunks of endplate that could damage membraneduring inflation. Much in the same way as drilling through plywood cancause a splintered edge around the perimeter of the newly created hole,there is some chance of creating protruding bone splinters when drillingthrough the endplate of the sacrum. It may reduce the chance of adverseinteraction between bone splinters and the membrane if these bonesplinters are broken off and removed.

1368—Insert 9 mm drill through cutter body bushing and drill into L5approximately ⅔rds of the way through the L5 vertebral body.

1374—Remove drill and cutter body bushing from 13 mm sheath.

1380—Insert 12.5 mm drill and create larger bore in L5 approximately ⅔rdof the way through the vertebral body. This may vary based on thepatient anatomy and the size of the distal anchor used.

1386—Remove drill and place guide pin with extension into bore.

1392—Pass 13 mm dilator body over wire and into sheath in order tofacilitate removal of the 13 mm sheath while making sure that guide pinremains in position.

1398—Select exchange system components based on the angle between thetrajectory and the anterior sacral face (choosing from component foreither 30 degrees or 45 degrees). As best seen in FIG. 3, the anteriorface of the sacrum is sloped. Angle 258 in FIG. 3 is approximately 40degrees. Not surprisingly, it is helpful to have a system thatapproximates the slope in the exchange cannula intended to contact theanterior face of the sacrum to establish an exchange cannula thatprotects components during insertion into the access channel.

FIG. 15 shows a perspective view of exchange cannula 704. The exchangecannula 704 has a handle 708, a main cannula 712 that runs from thehandle to the angled distal face 716. In this case the distal face 716is sloped at 45 degrees. A wire tube 720 runs along one wall of theexchange cannula 704 and through the handle 708 so that the exchangecannula 708 can be pinned to the sacrum to prevent the exchange cannulafrom sliding down the anterior wall of the sacrum, as described anddisclosed in co-pending and commonly assigned U.S. patent applicationSer. No. 11/501,351 filed Aug. 9, 2006, herein incorporated by referenceinto this disclosure.

1404—Insert chosen exchange bushing over the guide pin into the sacralbore.

1410—Pass chosen exchange cannula over exchange bushing making sure thatthe distal face of the exchange cannula is flush to the anterior face ofthe sacrum.

1416—Insert fixation wire through wire tube 720 in the exchange cannula704 and into sacrum using extended pin vise. (Pin vise allows the pin tobe fed through the vise and the vise tightened in order to give thegloved surgeon something larger to hold and manipulate than the wireitself). Those of skill in the art will be familiar with a Kirschner pinvise. A pin vise found to be suitable for this use is sold as partnumber 30008 by IMEX™ Veterinary, Inc. of Longview, Tex. One may want toextend the snout on the front of the pin vise to adjust the pin vise forthis particular application.

1422—Bend fixation wire securing exchange cannula 704 to sacrum (FIG. 3,element 116) and remove exchange bushing.

1428—End of access channel preparation.

After the access channel is prepared, the process of delivering a spinalmotion preservation assembly as shown in FIGS. 3-7 proceeds as describedin FIG. 14.

Components in the Dual Anchor Driver

In order to understand the steps in flow chart shown in FIG. 14, it willbe necessary to introduce and describe the components in various driversused in the process. The first driver is the dual anchor driver 2000.This driver and the components of the driver are FIGS. 16-22.

FIG. 16 is a perspective view of the dual anchor driver with a quarterround removed to better show the components. The operation of the dualanchor driver 2000 will be better appreciated after review of the use ofthe driver in the various steps in FIG. 15, but FIGS. 16-22 provide anintroduction.

A retention rod 2004 runs through the center of the dual anchor driver2000. Rotation of the retention rod knob 2032 causes the threaded distaltip 2036 of the retention rod 2004 to rotate relative to the dual anchordriver 2000 to selectively engage or disengage the internal threadedsection 368 of the distal bone anchor 340. FIG. 17 is a perspective viewof retention rod 2004 and shows the retention rod knob 2032 and thethreaded distal tip 2036.

The retention rod extends through an insertion tip 2008. Details of theinsertion tip may be seen in FIGS. 18 and 19. FIG. 18 is a perspectiveview with a quarter round removed. FIG. 19 is a cross section. In thiscase, the insertion tip 2008 has a hexagonal driving section 2040. Thoseof skill in the art will recognize that other shapes could be used forthe insertion tip. Note that distal to the driving section 2040 is anon-driving section 2044 which will be referenced below.

The insertion tip 2008 is engaged with the driver shaft 2012. Through acombination of a corresponding male hexagonal proximal end 2048 of theinsertion tip 2008 and a female hexagonal distal end 2052 (shown best inFIG. 20) and a pinned engagement, the insertion tip 2008 may be drivenby the driver shaft 2012 when the driver shaft 2012 is rotated by thehandle 2028.

The driver shaft 2012 lies within distraction sleeve 2016. Distractionsleeve 2016 may translate or rotate relative to driver shaft 2012 asreferenced below. FIG. 23 shows a side view of the distraction sleeve2016. FIG. 24 shows a perspective view with a quarter round sectionremoved. The distraction sleeve 2016 had a set of male threads 2056 onthe distal end of the distraction sleeve 2016 and another set of malethreads 2064 on the proximal end of the distraction sleeve 2016. Atextured section 2060 facilitates holding or moving the distractionsleeve 2016 while wearing surgical gloves. A set of female threads 2068in the proximal end of the distraction sleeve 2016 will be discussed inconnection with the distraction steps.

FIG. 21 is a cross section view of retainer lock 2020. Retainer lock2020 has a threaded section 2072 for engagement with the proximal femalethreads 2064 on the distraction sleeve 2016 (See FIG. 23). Retainer lock2020 also has an unthreaded section 2076 to allow the retainer lock 2020to move a fixed amount relative to the flange 2080 of lock stop 2024 asshown in FIG. 22. Lock stop 2024 is welded to the handle 2028.

Returning now to the flow chart in FIG. 14, the first step in thedelivery of a spinal motion preservation assembly of the type depictedin FIGS. 3-7 is step 1504.

1504—Load the proximal bone anchor 344 in the dual anchor driver 2000and engage the threaded section 2056 of distraction sleeve 2016 (SeeFIG. 23) into the set of internal threads 416 in the proximal boneanchor 344 to secure the proximal bone anchor 344 to the dual anchordriver 2000. Lock stop 2024 is welded to handle 2028 and thus constrainsthe movement of retainer lock 2020.

1510—Engage the internal threads 2072 on retainer lock 2020 withexternal threads 2064 on the distraction sleeve 2016.

1516—Place the distal bone anchor 340 on the dual anchor driver 2000being careful to align the distal bone anchor 340 with the proximal boneanchor 344 anchors using the alignment marks 472 placed longitudinallyon the external threads 356 and 404 of each anchor.

1522—Thread the distal tip 2036 of the retention rod 2004 by rotatingthe retention rod knob 2032 to cause the retention rod 2004 to rotaterelative to the outer layers of the dual anchor driver 2000 and thedistal bone anchor 340 and to engage internal threaded section 368 ofthe distal bone anchor 340.

1528—Place dual anchor driver 2000 with both anchors attached over theguide pin with extension and through the main cannula 712 in theexchange cannula 704 and begin rotating the driver 2000 to axiallyadvance the anchors into the distal and proximal vertebral bodies.Lateral fluoroscopy may be useful for visualization of the insertionsteps. Initially, the external threads 356 on the distal bone anchor 340will create a helical thread path in the proximal vertebral body 308 andat some point the distal bone anchor 340 will be creating a helicalthread path in the distal vertebral body 304 while the external threads404 on the proximal bone anchor 344 engage with the previously createdhelical thread path in the proximal vertebral body 308 as the twoanchors are being delivered by timed delivery based on a combination ofthe spacing of the anchors on the dual anchor driver 2004 and the use ofthe alignment marks 472.

1534—Continue rotating dual anchor driver 2000 to advance the distal andproximal bone anchors into distal and proximal vertebral bodies untilthe proximal face 370 (FIG. 5) of the distal bone anchor 340 isapproximately flush with the endplate on the distal end of theintervertebral disc space 312.

1540—Release the distal bone anchor 340 from the dual anchor driver 2000by rotating the retention rod knob 2032 and removing the retention rod2004 from the dual anchor driver 2000.

1546—Unscrew the retainer lock 2020 on the dual anchor driver 2000 torelease the external threads 2064 on the distraction sleeve 2016.

1552—Partially withdraw the dual anchor driver 2000 from the axialchannel 212 so that the driving section 2040 of the insertion tip 2008is no longer engaged with the hex ridges 374 in the distal bone anchor340 but remains engaged with driver engagement section 378 in theproximal bone anchor 344 (as shown in FIGS. 4-6). The non-drivingsection 2044 of the insertion tip 2008 may remain in the cavity 364(FIG. 5) in the distal bone anchor 340 as that will not impart rotationto the distal bone anchor 340.

1558—Selectively adjust the position of the proximal bone anchor 344.Often this may require advancing the proximal bone anchor 344 furtherinto the proximal vertebral body 308 (in this case the sacrum). It mayrequire retracting the proximal bone anchor 344 in some instances.Selective use of insertion tips of different axial lengths can beemployed to cause the timed delivery of the two bone anchors toinitially have a particular spacing between anchors. (normally aninteger multiple of the distance between adjacent external threads). Thedistance selected as the initial distance between bone anchors may beclinically indicated by the specific motion segment receiving therapyand the size of the patient's intervertebral disc space. An initialspacing of approximately 10 millimeters may be appropriate for somepatients receiving therapy to the L5/S1 motion segment.

After disengaging the distal bone anchor 340 from the dual anchor driver2000, the proximal bone anchor 344 can be advanced or retracted to beany selected distance from the distal bone anchor. The proximal boneanchor 344 may be positioned to have the distal face 414 (FIG. 4) of theproximal bone anchor 344 positioned approximately flush with theendplate of the proximal vertebral body 308 (in this case the sacrum).

1564—Remove the dual anchor driver 2000 with the exception of thedistraction sleeve 2016.

1570—Remove the extended guide pin.

The Distraction Driver and the Membrane Sheath

The next series of steps are performed through use of a distractiondriver. As with the discussion of the steps associated with the dualanchor driver 2000, it is useful to start with a description of thedriver and then discuss how the driver is used in context with thedelivery process that ensures protection of the membrane during (intact)deployment.

FIG. 25 is a perspective view of distraction driver 2100 with a quarterround removed. The distraction driver 2100 has distraction shaft 2104,distraction driver tip 2108, distraction driver sleeve 2112, and sizingsleeve 2116. This driver uses the same type of handle 2028 that wasintroduced in connection with the dual anchor driver 2000. This handleis used in a number of devices. Distraction driver 2100 has a membranesheath 2120 that is advanced to cover a membrane 2152 attached to amembrane tip 2148 by two retainer rings 2156 (See FIG. 26). Thismembrane 2152 is to be delivered to the intervertebral disc space (see312 in FIG. 3) between the two implanted anchors and then filled. Themembrane sheath 2120 is shown in FIG. 25 in the advanced positioncovering the membrane tip assembly 2132. The membrane sheath 2120 may beretracted by pulling on membrane sheath ring 2128 as the membrane sheath2120 moves along two membrane sheath keys 2124 (only one visible here).The distraction driver 2100 includes proximal O-ring 2136 and distalO-ring 2144. Thumbscrew 2140 will be discussed in more detail below.

1576—Assemble distraction driver 2100. This step includes the threadingthe membrane tip assembly 2132 onto distraction driver tip 2108. Asshown in FIG. 27, a perspective view with a quarter round removed of themembrane tip 2148 and FIG. 28 a cross section of the membrane tip 2148in conjunction with FIGS. 29 and 30 showing a perspective view with aquarter round removed and a cross section of the distraction driver tip2108. The engagement of the membrane tip assembly 2132 with thedistraction driver tip 2108 occurs through engaging the external threads2160 on the distraction driver tip 2108 with the internal threads 2164.

After the membrane tip assembly 2132 is engaged with the distractiondriver tip 2108, the membrane sheath 2120 may be advanced to cover thepreformed membrane 2152. This step may be facilitated by an optionalstep of pulling a vacuum at the proximal end of the handle 2028. Whilethe central cavity 2184 in distraction driver 2100, may not besufficiently air tight to able to hold a vacuum, this process will tendto remove air from the membrane 2152. A biocompatible lubricant that isalso compatible with the membrane 2152 may be used to help the processof extending the membrane sheath 2120 over the membrane 2152.

Step 1576 includes engaging the thumbscrew 2140 to secure distractiondriver sleeve 2112 to distraction shaft 2104 by engaging an indentation(visible in FIG. 25) in the distraction shaft 2104.

1582—Insert the distal end of distraction driver 2100 throughdistraction sleeve 2016 left in the axial channel 212 after delivery andpositioning of the two bone anchors (340 & 344). The insertion continuesuntil the distal end of the distraction driver 2100 arrives in thedistal bone anchor 340. The external threads 2056 on distraction sleeve2016 remain engaged with the internal threads 416 in the proximal boneanchor 344.

1588—Pull the membrane sheath ring 2132 to move the membrane sheath 2120to the retracted position. The membrane sheath ring 2132 is pinned totwo membrane sheath keys 2124 which are in turn connected via pins tothe proximal end of membrane sheath 2120. The membrane sheath 2120 maybe retracted now as it has served its purpose of protecting the membrane2152 from any possible damage while moving to the intervertebral discspace 312.

1594—While holding the textured section 2060 of the distraction sleeve2016 stationary with one hand, turn distraction driver 2100 untildesired distraction is achieved. The distraction is achieved by thethreaded engagement of the internal threads 2068 of the distractionsleeve 2016 (which are held stationary) with the external threads 2168on the distraction driver 2100. The rotation of the distraction driver2100 causes axial advancement of the distraction driver 2100 so that thedistal end of the membrane tip 2148 pushes on the interior cavity of theimplanted distal bone anchor 340 while rotating relative to the distalbone anchor 340. Thus, the distraction driver 2100 moves axially forwardrelative to the distraction sleeve 2016 which is threadedly engaged withproximal anchor 344 which is in turn threadedly engaged with proximalvertebral body 308. This movement causes the distal vertebral body 304to move axially and away from the proximal vertebral body 308 and thusincreases the size of the intervertebral disc space 312. Note that adistraction driver could be created that engages directly with the setof internal threads 416 in the proximal bone anchor 344. (U.S. patentapplication Ser. No. 11/259,614 filed Oct. 25, 2005 (published as US2006/0155297 A1) and referenced above includes a description of such adistraction driver and the relevant disclosure is incorporated herein byreference.) The use of the distraction sleeve 2016 allows a longer setof internal threads 2068 to be used by the distraction driver while thedistraction sleeve 2016 is connected to the internal thread 416 in theproximal bone anchor 344. Effectively the distraction sleeve 2016 is adistraction range extender as is it increases the maximum possibledistraction beyond the length of the internal threads 416 in theproximal bone anchor 344. If additional mechanical advantage is desired,the thread pitch in internal threads 2068 in distraction sleeve 2016could be made with a greater number of threads per inch than used ininternal threads 416 so that it takes a greater number of turns of thedistraction driver for a given amount of distraction and a proportionatedecrease in the amount of required force.

1600—Assemble and prepare prosthetic nucleus material delivery tools. Inthis instance the prosthetic nucleus material is silicone. The dualchamber silicone container is attached to an injection dispenser (notshown). A mixing tip (not shown) is threaded to the internal threads2172 in handle 2028 of distraction driver 2100 (see FIG. 25). Thesilicone dispenser is attached to the mixing tip.

1606—Inject material through distraction driver 2100 and into themembrane 2152 implant under real-time fluoroscopic imaging. Note thatthe distraction driver 2100 continues to hold the increased (distracted)spacing of the intervertebral disc space 312. The silicone flows throughthe mixing tip (not shown) which is threaded to the internal threads2172 in the back of the handle 2028 and up through the central cavity2184 of the distraction driver 2100 to the membrane tip 2148 and outthrough a set of one or more ports 2176. Beyond the ports 2176, themembrane tip 2148 is solid without any path for silicone.

Use pressure to drive silicone to fill membrane 2152 and cavity createdduring nucleectomy. Depending on the size of the cavity and the size ofthe selected membrane 2152, the volume of silicone placed in themembrane may not fully extend the membrane 2152 and there will bewrinkles or other irregularities in the surface of the membrane 2152. Insome cases the preformed shape of the membrane 2152 may be expanded bythe addition of silicone under pressure in order to fill the availablespace. However, the amount of expansion of the membrane 2152 will bemore like the expansion of a bladder in a soccer ball as it is inflatedthan the massive change in shape of a balloon as the membrane starts outas at least an approximation of the final shape of a filled membrane.The reduction in the amount of expansion of a membrane necessary to fillthe void left by the nucleectomy may make the less expanded membranemore resistant to damage from contact with sharp surfaces external tothe membrane. (For completeness, it is appropriate to note that the useof a membrane that starts essentially as a flat ring around the membranetip and expands under the pressure of the silicone application to fillthe void in the intervertebral disc space is a viable alternative to theuse of a preformed membrane described above.)

1612—Allow ample time for prosthetic nucleus material to cure. When thedistraction driver 2100 is removed, the silicone filled membrane 2152will be needed to maintain the distraction. (Some reduction indistraction may be seen as the distraction driver is removed leadingexperienced operators to optionally impose more than the desireddistraction with the distraction driver to anticipate the distractionreduction.) Do not proceed to the next step until the silicone is cured.

1618—Disengage distraction driver sleeve 2112 from distraction sleeve2016 by first unscrewing thumbscrew 2140 to release the engagementbetween the distraction driver sleeve 2112 and the distraction shaft2104. Once the distraction driver sleeve 2112 is free to rotate relativeto the rest of the distraction driver 2100, then rotate the distractiondriver sleeve 2112 to disengage the internal threads 2068 of thedistraction sleeve 2016.

1624—While holding distraction sleeve 2016 which is still engaged withthe proximal bone anchor 344, pull the distraction driver 2100 to pullthe membrane tip 2148 out of the distal bone anchor 340 and the proximalbone anchor 344 thus separating the membrane 2152 filled with siliconefrom the distraction driver 2100. As the membrane 2152 is attached tothe membrane tip 2148 and held by the retainer rings 2156, withdrawingthe membrane tip 2148 with the rest of the distraction driver 2100 whileleaving the membrane 2152 filled with cured prosthetic nucleus material(such as silicone) requires either ripping of the membrane 2152 at theretainer rings 2156 or pulling the membrane 2152 from under the retainerrings 2156 or some combination of ripping and pulling. Thus, removingthe distraction driver 2100 may require a fair amount of force.

The silicone will have cured within the membrane tip 2148 as well as outin the membrane 2152 and the silicone is apt to break at the ports 2176.Seal ring 396 (best seen in FIG. 6) helps keep the withdrawal of themembrane tip 2148 from introducing small pieces of silicone into theinternal threads 416 in the proximal bone anchor 344.

Turning now to FIGS. 31-36, the next driver is the distal cup driver2200. This driver is used to drive the distal cup 372 to engage theexternal threads 376 on the distal cup 372 with the internal threadedsection 368 of the distal bone anchor 340. The driver engages the hexridges 394 (best seen in FIG. 5). If the distal cup 372 is inserted intoa distal anchor 340 outside of the body then the driver design could berelatively simple. However, as the distal cup 372 must first bedelivered through the axial channel 212 before insertion into the distalbone anchor 340, it is useful to have the distal cup driver 2200 engagethe distal cup 372 to retain the distal cup 372 on the end of thedriver.

The distal cup driver 2200 is shown in FIG. 31 with expanding mandrel2204 on the end of the mandrel shaft 2224. FIGS. 32-36 provideadditional views of components in a distal cup driver 2200. An expansionplug 2208 is at the distal end of a plug shaft 2228 that runs throughthe center cavity of the mandrel shaft 2224. The external threads 2232on the proximal end of the plug shaft 2228 are threadedly engaged withhandle 2212. A knob 2216 has an internal cavity 2244 that receives theknob engagement section 2236. A screw 2220 can be inserted in theproximal end of the knob 2216 to engage internal threads 2240 in theknob engagement section 2236 of the plug shaft 2228.

1630—Engage the proximal end of the distal cup 372 with the distal endof the distal cup driver 2200 by aligning the hex ridges 394 in thedistal cup 372 with the expanding mandrel 2204. Turn knob 2216 clockwiseto cause the external threads 2232 on the proximal end of the plug shaft2228 to move relative to the handle 2212 to retract the expansion plug2208 and splay the expanding mandrel 2204 within the distal cup 372 sothat the distal cup 372 is retained by the distal end of the distal cupdriver 2200. While other designs for distal cups could be made withinthe teachings of the present disclosure, it is advantageous to have adistal cup driver that makes positive engagement with the distal cupwithout use of a retention tube that would engage a threaded cavitylocated in the bearing surface for the pivot as this would tend to leadto wear issues.

1636—Insert the distal cup 372 attached to the end of the distal cupdriver 2200 through the distraction sleeve 2016 (which is stillthreadedly engaged with the proximal bone anchor 344) and into thedistal bone anchor 340. Rotate the handle 2212 on the distal cup driver2200 to engage the external threads 376 on the distal cup 372 with theinternal threaded section 368 of the distal bone anchor 340 and advancethe distal cup 372 until seated. Once the change in resistance is feltas the distal cup 372 is seated, the rotation of the distal cup driver2200 should be stopped to avoid inadvertently advancing the distal boneanchor 340 in the distal vertebral body 304.

1642—Release the distal cup 372 from the distal cup driver 2200 byturning the knob 2216 counter-clockwise.

1648—Remove the distal cup driver 2200 from the axial channel 212 andthe distraction sleeve 2016.

The next driver is the support member driver 2260 shown in FIGS. 37 and38. FIG. 37 is a perspective view of a support member driver 2260 andFIG. 38 is a cross section of a support member driver 2260. Thecomponents of the support member driver 2260 are a support member drivershaft 2264 with a distal end 2268 of the support member driver shaft2264. In one implementation of the support member driver 2260, thedistal end 2268 of the support member driver shaft 2264 has the samebore diameter as the maximum cross sections of the distal end 384 andproximal end 388 of the support member 352 (in this implementation bothends are the same as best seen in FIG. 5). The distal end 2268 isadapted to receive the entire support member 352 within the supportmember driver shaft 2264. Because of the close dimensional tolerance,the support member 352 is engaged with the support member driver 2260and will not fall away from the driver until selectively expelled. Thesupport member driver shaft 2264 includes a textured section 2276 nearthe proximal end 2272 of the support member driver shaft 2264. A distalend 2288 of push rod 2280 may be inserted into the proximal end 2272 ofthe support member driver shaft 2264 and extended up to make contactwith the proximal end 388 of the support member 352. Pushing on thesupport member 352 with sufficient force by advancing the texturedsection 2284 of the push rod 2280 relative to the support member drivershaft 2264 will deliver the support member 352. With this introductionto this driver, the next step in the delivery process can beappreciated.

1654—Load the proximal end 388 of support member 352 into the distal end2268 of the support member driver shaft 2264 and continue to insert thesupport member until it is all within the support member driver shaft2264. The push rod 2280 may then be inserted into the proximal end 2272of the support member drive shaft 2264 and advanced without exertingforce to expel the support member 352.

1660—Insert the loaded support member driver 2260 into the axial channel212 through the distraction sleeve 2016 (which is still threadedlyengaged with the proximal bone anchor 344) and into the proximal boneanchor 344 until the shoulder 2292 on the support member driver shaft2264 bottoms out against the seal ring 396.

1666—Insert the distal end 392 of the support member 352 into the distalcup 372 by pushing the push rod 2280 towards the distal cup 372 whileholding onto the textured section 2276 of the support member drivershaft 2264.

1672—Remove the support member driver 2260 verifying through fluoroscopythat the distal end 392 of the support member 352 remains in positionwithin the distal cup 372.

1678—Place distal end of the dual anchor driver 2000 back through thedistraction sleeve 2016 so the driving section 2040 of the insertion tip2008 becomes engaged with driver engagement section 378 in the proximalbone anchor 344 in order to facilitate removal of the distraction sleeve2016.

1684—Turn distraction sleeve 2016 counter-clockwise after the insertiontip 2008 is engaged with the driver engagement section 378 and isholding the proximal anchor 344 stationary. After unthreading thedistraction sleeve 2016 from the proximal anchor 344, remove thedistraction sleeve 2016 with the dual anchor driver 2100 from theexchange cannula 704.

The next driver in the delivery process is a driver used to deliverfirst the proximal cup 420 and then the jam nut 440. The design of theproximal cup 420 and the jam nut 440 allow the use of a single driver.This is optional and other designs could use different drivers or atleast different tips on the driver to deliver these two components.

FIG. 39 is a perspective view of a dual use driver 2300. FIG. 40 is across section of the dual use driver 2300. From these two drawings, themajor components of the dual use driver 2300 can be introduced as: theinsertion tip 2304, the driver shaft 2308, a pair of retaining pins 2312(only one is visible) to hold the insertion tip 2304 to the driver shaft2308, a retention rod 2316, and a driver handle 2028. FIG. 41 shows anenlarged perspective view of insertion tip 2304 with the distal end inthe foreground. The distal end of the insertion tip 2304 is a polygonaldriver section 2324 which in this case is hexagonal and suitable toengage the driver engagement section 428 in proximal cup 420 (best seenin FIG. 7) or the driver engagement section 448 in jam nut 440 (bestseen in FIG. 7). FIG. 41 also shows one of the two pin engagement holes2328 used for engagement with a retaining pin 2312 (see FIG. 39). FIG.41 also shows that the distal face 2332 of the insertion tip 2304 has acentral bore 2336. FIG. 42 shows a perspective view of retention rod2316 including knob 2340 and threaded tip 2344.

A related component is the proximal anchor stabilizer 2380 shown in aperspective view in FIG. 43 and in a side view in FIG. 44 with anenlarged perspective view of stabilizer tip 2392 in FIG. 45. Theproximal anchor stabilizer 2380 has a stabilizer tip 2392 (having thestabilizer fingers 2384) connected by one or more stabilizer pins 2396(for example two pins) to the stabilizer shaft 2388.

1690—Place proximal anchor stabilizer 2380 over the shaft of the dualuse driver 2300 with the engagement fingers 2384 facing towards thedistal end of the dual use driver 2300.

1696—Place the proximal cup 420 onto the polygonal driver section 2324to engage the driver engagement section 428 in the proximal cup 420.

1702—Pass the threaded tip 2344 of the retention rod 2316 through thehandle 2028, driver shaft 2308, and the bore 2336 in the insertion tip2304 to make contact with the threaded cavity 432 in the proximal cup420. Engage the threaded tip 2344 with the threaded cavity 432 to securethe proximal cup 420 to the dual use driver 2300.

1708—Insert the proximal cup 420 and distal end of the dual use driver2300 through the exchange cannula 704 and begin threading the set ofexternal threads 424 on the proximal cup 420 (best seen in FIG. 5) intothe set of internal threads 416 in the proximal bone anchor 344 (whilemonitoring the fluoroscopes to ensure that the proximal anchor 344 isnot advancing further into the proximal vertebral body 308).

If proximal anchor 344 begins to rotate and thus advance at any pointduring proximal cup insertion or secondary distraction (discussedbelow), the proximal anchor stabilizer 2380 may be advanced axially toengage the stabilizer fingers 2384 with the slots 456 in the proximalbone anchor 344.

1714—Continue advancing the proximal cup 420 until it contacts theproximal end 388 of the support member 352.

1720—Selectively advance the proximal cup 420 beyond initial contactwith the support member 352 to impose a desired amount of secondarydistraction on intervertebral disc space 312 as continued advancementwill move the proximal cup 420 axially relative to the proximal boneanchor 344 which is being held in place by the proximal anchorstabilizer 2380 (should the anchor be inclined to move). The proximalbone anchor 344 is engaged with the proximal vertebral body 308. Thus,as the proximal cup 420 continues to advance, the proximal cup 420pushes the support member 352 axially. The support member 352 in turnpushes on the distal cup 372 in the distal bone anchor 340 in the distalvertebral body 304 to move the distal vertebral body 304 relative to theproximal vertebral body 308 to impose additional distraction.

The selective advancement of the proximal cup 420 alters the compressiveforces borne by the support member 352 as opposed to the silicon filledmembrane 2152. In an extreme case where the proximal cup is placed sofar away from the distal cup so that the support member cannot makecontact with both cups, the support member bears no compressive forceand all the compressive force must pass through the silicone filledmembrane 2152. Under sufficient compressive loading, the elasticallydeformable silicone filled membrane compresses in the Z axis whilebulging radially until the support member 352 makes contact with boththe proximal cup 420 and the distal cup 372 so that some of thecompressive force passes through the support member 352.

If in contrast that proximal cup 420 is advanced far enough, theincreased distraction of the intervertebral disc space may cause thesilicone filled membrane 2152 to lose contact with one or both vertebralbodies.

As the surgery is performed with the patient in the prone position(which facilitates distraction), the loading of the motion preservationassembly will change when the patient assumes a non-horizontal position.The surgeon adjusting the distribution of loading during the delivery ofa spinal motion preservation assembly may wish to anticipate the loadingchange when the patient eventually assumes a vertical orientation.

1724—Turn the knob 2340 on the retention rod 2316 counter-clockwise torelease the threaded tip 2344 from the proximal cup 420.

1730—Remove the dual use driver 2300 and the proximal anchor stabilizer2380 from the exchange cannula 704.

1736—Leaving the proximal anchor stabilizer 2380 on the dual use driver2300, place the jam nut 440 on the polygonal driver section 2324 of theinsertion tip 2304.

1742—Pass the threaded tip 2344 of the retention rod 2316 through: thehandle 2028, driver shaft 2308, and the bore 2336 in the insertion tip2304 to make contact with the threaded cavity 452 in the jam nut 440.Engage the threaded tip 2344 with the threaded cavity 452 to secure thejam nut 440 to the dual use driver 2300.

1748—Insert the jam nut 440 and distal end of the dual use driver 2300through the exchange cannula 704 and begin threading the set of externalthreads 444 on the jam nut 440 (best seen in FIG. 5) into the set ofinternal threads 416 in the proximal bone anchor 344 (while monitoringthe fluoroscopes to ensure that the proximal anchor 344 is not advancingfurther into the proximal vertebral body 308).

If proximal anchor 344 begins to rotate and thus advance at any pointduring insertion of the jam nut 440, the proximal anchor stabilizer 2380may be advanced axially to engage the stabilizer fingers 2384 with theslots 456 on the proximal bone anchor 344.

1754—Continue advancing the jam nut 440 until it is tight against theproximal cup 420.

1760—Turn the knob 2340 on the retention rod 2316 counter-clockwise torelease the threaded tip 2344 from the jam nut 440.

1766—Remove the dual use driver 2300 and the proximal anchor stabilizer2380 from the exchange cannula 704.

1772—Verify completion of steps with fluoroscopy and remove exchangecannula 704 after first removing the fixation wire connecting the wiretube 720 of the exchange cannula 704 to the sacrum 116.

1778—Close the axial channel between the sacrum and the skin throughconventional means.

ALTERNATIVE IMPLEMENTATIONS

Introduction of elastically deformable components.

The example shown in FIGS. 3-7 does not include an elastomeric componentor another elastically deformable component such as a spring thatprovides for elastic deformation of the spinal motion preservationassembly during a compressive load asserted in the Z direction. (Sealring 396 while likely to be elastomeric, is not positioned in theexample shown in FIGS. 3-7 in such a way to provide this functionality).

Machined Springs

FIG. 46 illustrates a cross section of a support member 900 with: afirst end 904, a second end 908, and a machined hollow rod 912connecting the two ends. The machined hollow rod has a section that is amachined spring 916. Springs machined into hollow rods (as opposed tosprings created from coil stock) can be made with great precision whichdecreases the variation between machined springs.

Elastomeric Component is Part of the Distal Cup

FIGS. 47-50 show a modified distal cup 2372 that includes an elasticallydeformable component in the form of a machined spring 2376. FIG. 47 is aside view of the modified distal cup 2372. FIG. 48 is a cross section ofthe same distal cup 2372. FIG. 49 is a perspective view with theproximal end of the distal cup 2376 in the foreground. FIG. 50 is aperspective view of the distal cup 2376 with the distal end in theforeground.

Distal cup 2372 may be fabricated to have the same cavity 380 in thedistal cup 2372 to receive the distal end 384 of support member 352 andhave the same hex ridges 394 to be engaged by the distal cup driver 2200described above. The distal cup 2372 may be created to have the sameexternal threads 376 on the distal cup 2372 to engage the distal anchor340 in the same way as distal cup 372 discussed above. The added featurein distal cup 2372 is the ability to undergo elastic compression alongthe Z-axis as the machined spring 2376 elastically compresses. As themachined spring 2376 may be created to be substantially stiff inresistance to torsion, the presence of the machined spring 2376 may notbe noticed during the delivery of the distal cup 2374 to the distalanchor 344.

FIG. 51 shows a perspective view with a partial quarter round removed ofalternative distal cup 2400. Distal cup 2400 has a cup section 2404 thatreceives the distal end 384 of the support member 352. The cup section2404 is capable of a limited range of movement axially relative to anouter sleeve 2416 and a tiered distal segment 2420 that fits up into adistal anchor 344 (excluding the internal threaded section 368 (see FIG.4)). An O-ring 2412 may be placed around the cup section 2404 adjacentto the outer sleeve 2416.

As the cup section 2404 moves towards the tiered distal segment 2420,the alignment rod 2424 section of the cup section 2404 moves within acavity in the tiered distal segment 2420. Comparing FIG. 51 to FIG. 52,elastomeric ring 2408 is elastically deformed as the cup section 2404moves towards the tiered distal segment 2420. When the elastomeric ring2408 substantially fills the ring cavity 2428, resistance to axialmovement is greatly increased and substantially stops further axialmovement. In FIG. 52, the elastomeric ring 2408 is shown as a darkmaterial and fills the ring cavity 2428. Notice that the elastomericring 2404 and ring cavity 2428 may be selected relative to the othercomponents so that the ring cavity 2428 is filled before the cup section2404 bottoms out on the tiered distal segment 2420 as indicated by gap2432.

The elastomeric ring 2404 may be made from a variety of semi-compliantmaterials that are appropriate for insertion into a human body such asby way of examples fluoropolymer elastomer (Viton™), polyurethaneelastomer, or silicone rubber.

There are a number of options for delivery of the distal cup 2400. Hexridges such as ridges 394 shown in FIG. 5 could be added so that thedistal cup driver 2200 discussed above could be used. Having the samehex ridges so that distal cups with and without elastomeric componentscould be used with one common driver as deemed expedient with asurgeon's judgment for treating a particular patient would simplify thetool kit.

A set of detent concavities could be placed in the perimeter of thecavity in the cup section which can be selectively engaged by a detentprotrusion of a corresponding driver to engage the distal cup to deliverthe distal cup to the interior of the distal bone anchor. A descriptionof such detent concavities and a corresponding driver appears inpreviously filed U.S. patent application Ser. No. 11/256,810subsequently published as US 2006/0079898 A1 (See for example FIGS. 18and 19 of the '810 application and the relevant text). The relevantportions of that published application are hereby incorporated byreference.

Yet another delivery option would be a delivery tool analogous to thesupport member driver 2260 which would receive at least a proximalportion of the distal cup in a shaft and then discharge the distal cupwith an appropriate push rod to deliver the distal cup to the distalbone anchor.

Other Possible Locations for Elastic Component

Another option (not shown in the figures) for the use of an elasticallydeformable component in order to aid the ability for compressive axialtranslation and load distribution is to place an O-ring, elastomericwasher, or other elastomeric object or spring between the distal end ofthe distal cup 372 and the distal bone anchor 340. In order to allow forelastic deformation of the spinal motion preservation assembly, thedistal cup would need to be able to move relative to the distal anchorso a threaded engagement would not be appropriate. Ideally, the distalcup or the distal bone anchor, or both would be shaped to allow spacefor the elastomeric material. Such elastomeric components may beconfigured from semi-compliant materials, for example fluoropolymerelastomer (Viton™), polyurethane elastomer, or silicone rubber.

Another location that could be used for the placement of an elasticallydeformable component is within the proximal bone anchor between theproximal cup and the jam nut (example not shown). In this configurationthe proximal cup would not have external threads so that it would befree to move axially in the cavity of the proximal bone anchor tocompress the elastically deformable compound between the proximal end ofthe proximal cup and the distal face of the jam nut. Yet anotherlocation for placement of the elastically deformable component isbetween the bearing surface and the pivot.

One of skill in the art will recognize that when using two elasticallydeformable inserts in a single motion preservation assembly, the insertscould have different properties such as being made of differentthicknesses or from different materials so that one responded underlower axial loads than the other.

FIG. 65 illustrates an embodiment of a spinal implant assembly 4000 withat least one elastomeric component placed within the spinal implantassembly 4000. Many of the components in FIG. 65 are analogous tocomponents discussed above such as in connection with FIGS. 3-7.

A distal bone anchor 4340 has a distal face 4366 and a proximal face4370. A cavity 4364 runs along the longitudinal axis of the distal boneanchor 4340 to allow deployment over a guide wire. The cavity 4364accessible from the proximal face 4370 receives a distal elastomericinsert 4550 such as an O-ring. The cavity 4364 is shaped to receive thedistal elastomeric insert 4550 without allowing the distal elastomericinsert 4550 to pass out the distal face 4366.

A distal cup 4372 with a distal face 4376 fits into the cavity 4364 andrests against the distal elastomeric insert 4550. Whereas otherembodiments may use a threaded distal cup that engages a threaded cavityin the distal anchor, this distal cup 4372 has some freedom to movewithin the cavity 4364 in the distal/proximal direction (Z-direction) toelastically deform the distal elastomeric insert 4550.

A support member 4352 has: a distal end 4384, a proximal end 4388, and asupport member body 4392 located between the distal end 4384 and theproximal end 4388. The distal end 4384 of the support member 4352 isreceived in a cavity 4380 in the proximal end of the distal cup 4372 tosupport relative movement as described above.

A proximal cup 4420 with a distal cavity 4436 open on the distal end ofthe proximal cup 4420 receives the proximal end 4388 of the supportmember 4352 to support relative motion as described above. The proximalface 4428 of the proximal cup 4420 is positioned in the cavity 4412 inthe proximal bone anchor 4344.

A proximal elastomeric insert 4554 fits within the distal end of cavity4412 between the proximal bone anchor 4344 and the proximal face 4428 ofthe proximal cup 4420. The proximal face 4428 of the proximal cup 4420being substantially planar. The proximal cup 4420 having some ability tomove in the distal/proximal (Z-direction) to elastically deform theproximal elastomeric insert 4554 within proximal bone anchor 4344.

The cavity 4412 has a set of internal threads 4416 at the proximal endof the proximal bone anchor 4344.

A threaded cap 4440 with a set of external threads 4444 suitable toengage the set of internal threads 4416 in the proximal bone anchor4344. The threaded cap 4440 having a driver engagement section 4448 in acavity open at the proximal face 4450 of the threaded cap 4440. Thethreaded cap 4440 optionally including a threaded cavity 4452 (notvisible in FIG. 65) to interact with a driver or retaining rod.

As illustrated in FIG. 4 a motion preservation assembly using thehardware show in FIG. 65 would include a membrane 460 filled withprosthetic nucleus material 464. To highlight this concept, FIG. 65includes a preformed membrane from FIG. 53 (discussed below) showing aperspective view with quarter round removed of a preformed membrane2450.

Process to Deliver a Spinal Motion Preservation Assembly

FIG. 66 is a high level flow chart of a process to deliver a spinalmotion preservation assembly of the type shown in FIG. 65. As theprocess to deliver a spinal motion preservation assembly has beendiscussed above in connection with FIGS. 12-14, this discussion can bebrief.

5106—Create axial channel 212 (See FIG. 3) in the manner describedabove.

5112—Deliver both bone anchors (4340 & 4344) to the pair of vertebralbodies and adjust the position of the distal bone anchor 4340 relativeto the distal vertebral body, 304. As described above, the two anchorsmay be adapted so that they may be delivered by timed delivery on asingle dual anchor driver. Optionally, the dual anchor driver may use aretention rod to engage an internal threaded section (not shown) in thedistal bone anchor 4340.

5118—Adjust the position of just the proximal bone anchor 4344 afterdisengaging the dual anchor driver from the distal bone anchor 4340.

5124—Distract the intervertebral space 312 (See FIG. 3) between thedistal vertebral body 4304 and the proximal vertebral body 4308 byforcing an increase in distance between the distal bone anchor 4340 andthe proximal bone anchor 4344.

5130—Add prosthetic nucleus material to fill the void in theintervertebral disc space 312 (See FIG. 3) but not a portion of theintervertebral disc space between the proximal bone anchor 4344 and thedistal bone anchor 4340 as that space is occupied during filling by aportion of the device used to fill the prosthetic nucleus.

5136—Allow the prosthetic nucleus material to cure so that theprosthetic nucleus can substantially maintain the distraction after thedistraction tool is removed and share some of the load applied to themotion segment.

5140—Deliver the distal elastomeric insert 4550 into the distal boneanchor 4340 such that it is in position to be compressed by the movementof the substantially planar distal surface 4376 of the distal cup 4372against a portion of the distal bone anchor 4340. In someimplementations, the delivery of the distal elastomeric insert 4550 maybe combined with the delivery of the distal bone anchor 4340 or with thedelivery of the distal cup 4372.

5142—Deliver the distal cup 4372 into the distal bone anchor 4340.

5148—Deliver the support member 4352 to place the distal end 4384 of thesupport member in proximity to the distal cup 4372.

5154—Deliver the proximal cup 4420 to the proximal bone anchor 4344.

5156—Deliver the proximal elastomeric insert 4554 to the proximal boneanchor 4344 so it abuts the proximal face 4428 of the proximal cup 4420.In some implementations, the delivery of the proximal elastomeric insert4554 may be done with the delivery of the proximal cup 4420.

5160—Deliver the threaded cap 4440 to the proximal end of the cavity4412 in the proximal bone anchor 4344 and engage the external threads4444 on the threaded cap 4440 with the internal threaded section 4416 ofthe proximal bone anchor 4344 until the distal end of the threaded capmakes contact with the proximal elastomeric insert 4550.

5166—Adjust load distribution between that as supported by supportmember (e.g., distributed to bone) and that amount as shared by theprosthetic nucleus (e.g., load transferred to the annulus fibrosus) bycontinuing to advance the threaded cap 4440 to push against the proximalelastomeric insert 4554 and thus moving the proximal cup 4420 relativeto the proximal bone anchor 4344.

5172—Close the axial channel.

Other Elastically Deformable Component Options.

Given an appropriate modification to the shapes of the components withinthe motion preservation assembly, the elastically deformable componentscould be springs of any one of various configurations and stiffness thatwould allow for elastic deformation without reliance on the use ofelastomeric components. A coil spring is one option. Another option isone of the various types of spring washer products such as a Bellevilledisc. Spring washers can be stacked to provide for greater totaldeflection or simply to change the response curve of deflection toforce.

Additional Discussion of Membranes

FIG. 53 shows a perspective view with a quarter round removed of apreformed membrane 2450 with a one inch diameter (as measured inside thepreformed membrane before adding silicone material). FIG. 54 shows across section of the same one inch preformed membrane 2450. To provide acontrast, FIG. 55 shows a cross section of a ¾ inch preformed membrane2460 that could be delivered by the same delivery device and membranetip 2148 but may be preferred by a surgeon working with a disc that hasa smaller diameter. FIG. 56 shows a detail applicable to both FIGS. 54and 55, the membrane channel engagement section 2454 of one side of amembrane. This membrane channel engagement section 2454 is placed withinone of the two membrane channels 2180 in a membrane tip 2148 (See FIG.27) and then held in place by a retainer ring 2156 (See FIG. 26).

FIG. 57 is a perspective view of an alternative membrane 2470. FIG. 58is a cross section of the same membrane 2470 shown in FIG. 57. Membrane2470 is a flat membrane and is expanded from flat into a substantiallyconforming fit within the available space in the intervertebral discspace. When using a flat membrane 2470, there remains some advantage tousing a distraction driver with a sheath to protect the flat membraneduring delivery to the intervertebral space but this is perhaps lessnecessary than when using a preformed membrane. Thus, a flat membranecould be delivered with a distraction driver that does not have a sheathto protect the flat membrane during movement to the intervertebral discspace. One of ordinary skill in the art could create a reviseddistraction driver based upon the disclosed distraction driver 2100 toremove the membrane sheath 2120, membrane sheath key 2124, membranesheath ring 2128 and relevant connection pins.

Membrane 2470, a highly expandable membrane, may be made of anelastomeric material, e.g., silicone rubber, such as that obtained fromNusil Silicone Technology located in Carpeneria, Calif., exhibiting acapacity for elongation of between about 500% and about 1500% and mostpreferably about 1000% and having a wall thickness of 0.220 inches.Membrane 2470 has both a distal membrane channel engagement section 2454and a proximal membrane channel engagement section 2454 that fit withinthe two membrane channels 2180 in the relevant membrane tip 2148. Afterretainer rings 2156 are place over the two membrane channel engagementsections 2154 to hold the membrane channel engagement sections in themembrane channels 2180 (preferably while compressing the retainer rings2156 to make the ring smaller), the membrane 2470 is firmly connected tothe membrane tip 2148 such that the membrane 2470 will stretchsignificantly to fill the intervertebral disc space under pressure frominserted prosthetic nucleus material without pulling free of themembrane tip 2148. When the membrane tip 2148 is removed after theinserted prosthetic nucleus material (such as silicone) has cured, themembrane 2470 disengages from the membrane tip 2148 as the membranechannel engagement channels 2454 rip, pull free, or some combination ofboth.

The flat membrane 2470 will undergo substantial expansion when filledwith prosthetic nucleus material. The perimeter of flat membrane 2470 atclose to the midline 2474 along the cephalad/caudal axis may increasefrom about 1.5 inches (diameter of 0.475 inches) to as much asapproximately 4 inches or more (diameter of approximately 1.25 inches).This would be an ending midline perimeter of more than 265% of theinitial perimeter length. Even if the flat membrane was used in a voidwith a smaller cross section, the midline perimeter 2474 may increase tomore than 2.3 inches (diameter of approximately 0.750 inches), a finalmidline perimeter 2474 length of more than 150% of the initial midlineperimeter length. Such substantial increase in the perimeter length andthe increase in surface area makes the expanded flat membrane moresusceptible to damage than the unexpanded flat membrane as the expansionthins the wall of the membrane.

The midline perimeter of the membrane as it would be delivered to thedisc space substantially aligned with the cephalad/caudal axis is likelyto be the portion of the membrane that expands the most when filling thevoid in the intervertebral disc space. However, one of skill in the artwill recognize that the void may not be uniform around thecephalad/caudal axis or the membrane may not be perfectly centered withthe void. Thus, the actual perimeter of the membrane that undergoes themaximum growth may be slightly above or slightly below the midlineperimeter. The teachings with respect to reducing the increase in sizeby using a preformed membrane instead of a flat membrane continue toapply.

When using a preformed membrane that approximates the size and shape ofthe void to be filled, the preformed membrane may not need to expand atall. Even when it does expand, the expansion will be on a much smallerscale. For example, when using a preformed membrane such as preformedmembrane 2450 in the same void discussed above that requires expansionto a 1.25 inch outer diameter, the midline perimeter 2458 would changefrom approximate approximately 3.25 inches (1.040 inch diameter) toapproximately 4 inches of midline perimeter (1.25 inch diameter). Thiswould be a final midline perimeter of less than 125% of the initialmidline perimeter.

Even if a preformed membrane with a 0.750 inch internal diameter such aspreformed membrane 2460 were used in a intervertebral disc space with avoid requiring expansion to 1.0 inches of diameter, the increase inmidline perimeter 2464 would increase from approximately 2.5 inches(0.790 inches of initial outside diameter) to approximately 3.14 inches(diameter of approximately 1 inch), this would be a final midlineperimeter of a little more than 125% of the original midline perimeter.

Membrane Free Alternative

In yet another alternative implementation, the membrane tip analogous to2148 can be used without any membrane at all. In this alternativeimplementation, the prosthetic nucleus material 464 (FIG. 4) isintroduced directly into the intervertebral disc space, instead offilling a membrane. The injection of prosthetic nucleus material 464directly into the intervertebral disc space 312 may be performed eitherwith or without a preceding sealant step to seal the surfaces of theintervertebral disc space by means of materials and methods such asdescribed in co-pending and commonly assigned U.S. patent applicationSer. No. 11/199,541 filed Aug. 8, 2005, and subsequently published as US2006-0206209 A1, the relevant material on sealants incorporated byreference.

In summary, in prosthetic nucleus motion preservation assemblyembodiments configured without expandable membranes, generally atwo-step deployment process is used wherein a barrier-sealant membrane(BSM) is preferably first introduced through conformal contact with theinterior surfaces of the intervertebral disc space to seal physiologicstructures, e.g., fissures in the annulus, to preclude leakage of thesubsequently introduced bulk prosthetic nucleus material. For prostheticnucleus devices used as part of a motion preservation assembly, theviscoelastic properties, e.g., bulk and compressive moduli, are designedto substantially match those of the native disc nucleus, to functionallyenable conformal contact of maximum device surface area within theintervertebral disc space; to mimic physiologic load distribution anddissipation; prevent bone erosion or implant subsidence; and exhibitsufficient resistance to fatigue and shear forces to preclude materialfragmentation and migration out of the disc. In embodiments where themotion preservation assemblies are configured for use in conjunctionwith a barrier-sealant membrane, the barrier-sealant may include aqueoussolutions of synthetic or purified (non-antigenic) biopolymers orproteins, such as collagen or collagen-albumin mixtures or slurries; orfibrinogen, thrombin, and the like, or combinations thereof, of suitablyhighly fibrous; highly cross-linked; high density of solids (e.g., >65mg/ml). In one embodiment, it is preferred that the biopolymer proteinsystem be modified to be insoluble, and that proteins be of Type 1 whenpossible and appropriate. In another embodiment, the sealantadditionally may include a cross-linking agent, e.g.,gluteraldehyde/aldehyde, or other suitable functional groups modified tominimize toxicity and/or necroses (e.g., citric-acid derivative).

In a preferred aspect of barrier-sealant membranes, the cross-linkingagent(s) may include functionalities which reduce residuals or which arematerials that are naturally metabolized. In one embodiment, thecross-linking agent may include at least one citric acid derivative andsynthetic or highly purified biopolymer or protein, such as systems asjust described, (e.g., collagen; collagen-albumen; collagen; elastin,etc.). In a preferred aspect, the cross-linker is a relatively lowweight macromolecule which may include polar functional groups, such ascarboxyl groups or hydroxyl groups, that are modified by means ofelectron attracting groups, e.g., succinimidyl groups.

In yet another embodiment, the barrier-sealant and/or barriers (e.g.,thicker layers) may include hydrocolloids. More specifically, thebarrier-sealant membrane may be configured to include water solublehydrophilic colloidal components, e.g., carboxymethylcellulose, incombination with elastomers or biopolymers as sealants or tissue repairmatrices, respectively, and wherein the barrier membrane includesnon-degradable, semi-permeable film. In other embodiments, barriers maybe pectin-based or foam.

Suitable Materials

The design choices for suitable materials for the bone anchors allow theuse of titanium, cobalt chromium alloys, or possibly stainless steel.Those of ordinary skill in the art will recognize that other materialscould be used for the bone anchors.

With respect to the pivot that comes in contact with the bearingsurfaces and the bearing surfaces themselves, it may be useful to selecta material with superior wear resistance such as a cobalt-chromiumalloy, selected ceramics, stainless steel, MP35N, and possibly evenultra high molecular weight polyethylene (UHMWPE) for the cup butprobably not the pivot itself. Many would not consider titanium a goodchoice for these components. The support body between the pivot endscould be made from titanium, cobalt-chromium alloys, MP35N or othermaterials.

Moreover, the ends of the support member may be treated (e.g., surfaceor heat treatments as appropriate) to enhance wear resistance. Thus,while the chemical composition may be the same, the ends of the supportmember are now a different material from the middle as in this context“material” is a combination of the composition and treatments to produceproperties. In yet another implementation, the support member body 392may be made of another compatible material (e.g., one that will notreact so as to cause electrochemical corrosion) so that while thesupport member ends are made more wear resistant, the material used forthe support member body and or and/or the treatment applied to thesupport member may be treated to enhance fatigue resistance relative tothe material used for the support member ends. Having the support membermade from two ends and a body rather than machined from a single pieceof metal is not an unreasonable manufacturing strategy even if the endsand body are made from the same material, as highly polished, highlyround, spheres are available commercially.

The retainer rings may be made from a shape memory material such asNitinol. The retainer rings may also be made from a material such astitanium. Such titanium rings can be crimped onto the membrane tip 2148to hold the membrane in place and then the ends laser welded.

While one viable combination is to use membranes and prosthetic nucleusmaterials that are the same material so that the injected materialbecomes one solid nucleus with the membrane (such as occurs whensilicone is injected into a silicone membrane), this is not arequirement. Dissimilar materials may be used. When using preformedmembranes that are not expected to undergo significant expansion, awider range of materials may be used including membranes made ofsemi-complaint specialty fabric. A woven fabric may have theadvantageous property of allowing air to quickly bleed through thefabric during the filling process while retaining the prosthetic nucleusmaterial. (Normally the connection of the membrane to the membrane tip,though strong, is not air tight and air can bleed out the membranechannels.) The prosthetic nucleus material may fill voids in the fabricand such that the fabric becomes captured in the edge of the prostheticnucleus material before the material cures.

The membrane may be fabricated to have more than one layer. For example,an outer layer that serves to protect the inner layer from sharp edgeson bone fragments and an inner layer intended to retain the prostheticnucleus material. Thus a membrane with an external layer of a wovenfabric may contain an inner layer of a material such as silicone.

Additional Details on Retainer Rings

One retainer ring that may be used to retain the membrane is a Nitinolring, a nickel titanium alloy. As a shape memory alloy, Nitinol resumesa trained shape when it reaches a specific temperature. Cooling Nitinolin a bath of isopropyl alcohol (which may be cooled with dry ice) willallow the Nitinol ring to be expanded so that through a series ofcooling and pressing cycles, the Nitinol ring can be expanded and placedover the membrane.

Subsequent heating of the Nitinol rings will cause them to resume theiroriginal shape and retain the membrane. Application of an appropriateadhesive to the underside of the membrane channel engagement section2454 of a membrane before applying a retaining ring helps keep themembrane in position during the efforts to place the retaining ring overthe membrane.

ALTERNATIVES AND VARIATIONS Delivery to Motion Segment other than L5/S1

In order to provide concreteness to the disclosure provided above, aspecific motion segment was discussed. In this instance it was the L5/S1motion segment. While the dimensions of components may be slightlydifferent when implanted in a different motion segment, nothing in theabove disclosure should be interpreted as limiting the disclosure totherapeutic treatment of the L5/S1 motion segment. Other motion segmentsincluding by way of example and not limitation the L5/L4 motion segmentand the L3/L4 motion segment may benefit from delivery of a spinalmotion preservation assembly that uses one or more teachings from thepresent disclosure.

Adaptation of Sheathed Delivery to Deliver a Prosthetic Nucleus

A subset of the teachings in the present disclosure could be adapted todeliver an intact and undamaged prosthetic nucleus membrane to anintervertebral disc space. FIGS. 59 and 60 help depict this process.FIG. 59 shows a cross section of a spine with an implanted prostheticnucleus 2504 and FIG. 60 shows a membrane tip 2520 with a preformedmembrane 2524.

As shown in FIG. 59, an axial bore would be prepared through theproximal vertebral body 308 (such as the sacrum) and the intervertebraldisc space 312 would be prepared in keeping with this disclosure andsurgical needs of the specific patient. The distraction driver of thepresent disclosure could be fitted with a shorter single ring membranetip 2520 that uses a preformed membrane 2524 with a single opening andsingle membrane channel engagement section (not visible here but compareFIG. 56) engaged with a single retainer ring 2156. After the preformedmembrane 2524 is moved through the axial channel to the intervertebraldisc space 312, the sheath would be withdrawn to uncover the preformedmembrane 2524. The membrane tip 2520 could be positioned to be close tothe edge between the proximal vertebral body 308 and the intervertebraldisc space 312. Prosthetic nucleus material 464, such as silicone, couldbe delivered as described above with the difference that rather than aseries of lateral ports, a single distal port (not shown) would be usedto fill the preformed membrane 2524 to substantially conform to the voidin the intervertebral disc space 312. Using a preformed membrane 2524 ofapproximately the required size would reduce or eliminate the need toexpand the preformed membrane 2524 when filling the preformed membrane2524 thus making the membrane less susceptible to damage.

Subsequent to curing of the prosthetic nucleus material 464 and theremoval of the membrane tip 2520 from the axial channel, the withdrawalcausing the membrane attached to the cured prosthetic nucleus to ripand/or pull free from the retainer ring 2156, a stop flow means (forexample a bone plug 2530) could be delivered to plug the bore in theproximal vertebral body 308. In one implementation, the bone plug 2530may include external threads 2534 to engage the bone peripheral to theaxial bore in the proximal vertebral body. One could use an axial boreand bone plug 2530 of a smaller cross section than shown in FIG. 59. Forexample, an axial bore of 400 mils (0.400 inches) or less may besufficient to allow passage of a sheathed membrane and a membrane tip.The bone plug for an axial bore of 400 mils would tend to have a minordiameter for the threaded section of 400 mils or slightly more than 400mils. The bone plug 2530 could be made from an allograft, abiocompatible metal, a biocompatible polymer, or another appropriatesubstance.

One of skill in the art could make a delivery tool that deliversmembranes of this type without the capacity to perform distractionrather than use the distraction driver 2100 of this present disclosurewith a modified membrane tip. The discussion of this alternativeimplementation of some of the teachings of the present disclosure hasintentionally been held brief given the lengthy disclosure overall. Oneof ordinary skill in the art will recognize that the preformed membranecould be made of a woven fabric, a multilayer membrane, a materialdifferent from the injected prosthetic nucleus material and all of theother variations discussed in connection with the other prostheticnucleus membranes.

Preformed Prosthetic Nucleus

The disclosure has discussed the creation of a prosthetic nucleusthrough the delivery of flowable prosthetic nucleus material (with orwithout an external membrane) and the subsequent curing of theprosthetic nucleus material to form a prosthetic nucleus. Many teachingsof the present disclosure may be applied while using a set of one ormore previously formed prosthetic nucleus discs. The discs would bedelivered to the intervertebral disc space either in many thin layers orin thicker discs with a radial slit so that the split disc could bedelivered to the intervertebral disc space before reassuming a discshape. As long as the discs had an open center, the support member couldbe inserted as described above after insertion of the discs.

A disc could be inserted from a supplemental access route substantiallyorthogonal to the axis of the spine. (substantially orthogonal beingwithin 45 degrees of orthogonal) If done during distraction of themotion segment, a split disc could be inserted into the intervertebraldisc space to substantially encircle the distraction tool. After thedistraction tool was removed, the inserted disc material wouldsubstantially maintain the distraction and the process of delivering thecomponents could continue in keeping with the teachings set forth above.

Multilevel Spinal Motion Preservation Assemblies

While the implementations described in detail above are directed to aspinal motion preservation assembly providing therapy to a single motionsegment, this is not an inherent limitation for the teachings of thepresent disclosure.

For example, a spinal motion assembly could be implemented in accordancewith the teachings of the present disclosure wherein the spinal implantassembly includes: a means for anchoring the spinal implant assembly toa third vertebra located immediately adjacent to and more cephalad thanthe more cephalad of the first and second vertebrae, an additionalpivot-like means so that after the spinal implant assembly is anchoredto both of the first and second adjacent vertebrae and to the thirdvertebra that the third vertebra can move relative to the more cephaladof the first and second adjacent vertebra in addition to the morecephalad of the first and second adjacent vertebra being able to moverelative to the more caudal of the first and second adjacent vertebra.

Turning to FIG. 61, a two level spinal motion preservation assembly 3500is shown in a perspective view (with the membranes hidden to allow abetter view of the components). FIG. 62 shows the various componentsbeyond the three bone anchors and two membranes (membranes not shown).Visible in FIG. 62 are distal bone anchor 3504, medial bone anchor 3508,and proximal bone anchor 3512. In this particular implementation, themajor diameters of the distal bone anchor 3504, medial bone anchor 3508,and proximal bone anchor 3512 are the same and the three bone anchorsmay be delivered by a single delivery tool using timed delivery so thatthe thread path cut by the distal bone anchor 3504 is used by the medialbone anchor 3508 and the proximal bone anchor 3512. In a manneranalogous to that described above in connection with the delivery of twoanchors, the three anchors would be loaded onto the driver, perhapsusing alignment marks (not shown here but compare element 472 in FIG. 6)and delivered such that the distal bone anchor 3504 is placed in properposition. The driver would be removed from engagement with the distalbone anchor 3504 and then the position of the medial bone anchor 3508would be adjusted. The driver would be removed from engagement with themedial bone anchor 3508 and the placement of the proximal bone anchor3512 would be adjusted.

Turning now to FIG. 62, the components internal to the two-level spinalmotion preservation assembly 3500 are shown. The dual pivot for thedistal motion segment has a distal cup 3604, support member 3608, andproximal cup 3612. Seal ring 3616 which is placed in the medial anchor3508 is also shown. In addition to the seal ring 3616 and the proximalcup 3612, the medial anchor includes the distal cup 3620 for theproximal motion segment. The dual pivot for the proximal motion segmentincludes the distal cup 3620, the support member 3624, and the proximalcup 3628. The proximal anchor 3512 also includes seal ring 3632 and jamnut 3636.

When working with a two-level spinal motion preservation assembly, itmay be useful to carefully calculate the dimensions of the two motionsegments after the desired distractions so that components can becarefully selected from a range of components of different sizes thatthe assembly is sized appropriately to minimize the amount of adjustmentrequired.

The delivery of a two-level spinal motion preservation assembly differsfrom the discussion of FIGS. 3-7 in that in addition to what is done inconnection with FIGS. 3-7, the two-level spinal motion preservationassembly also anchors the spinal motion assembly to a third vertebralbody immediately adjacent to and more cephalad than the original distalvertebral body which is the most cephalad of the two original vertebralbodies. The two-level spinal motion preservation assembly includes anadditional pivot means so this third vertebral body is able to moverelative to the original distal vertebral body (which now becomes themedial vertebral body).

Alternatives to the Stabilizer/Slot Pair

The delivery sequence set forth above made use of a proximal anchorstabilizer 2380 with stabilizer fingers 2384 (See FIGS. 43-45) to engagewith slots 456 (FIG. 5). The teachings of the present disclosure shouldnot be limited to this specific finger and slot arrangement. Otherirregular surfaces on the proximal face 408 of the proximal anchor couldbe used as a point of engagement with a stabilizer tool. One of ordinaryskill in the art will appreciate that in this context an irregularsurface feature is not just a texture but an protrusion or cavity withinone or more surfaces accessible during process steps that mightinadvertently rotate the proximal anchor.

In order to provide concrete examples, specific handedness of screwthreads are shown in the figures and implied in the description of theprocess steps. One of ordinary skill in the art may alter all or some ofthe handedness of threads without departing from the teachings of thepresent disclosure.

Use of a Single Pivot

While the range of motion for a motion segment with an installed spinalmotion preservation assembly having a bearing surface in each of the twobone anchors (as shown in FIGS. 9( c) and 9(d) may allow for a greaterrange of motion for the motion segment than a single pivot (compareFIGS. 9( a) and (b)), nothing should be interpreted as limiting thedisclosure to a double pivot or a specific form of a double pivot unlessthe limitation is explicit in the claims as spinal motion preservationassemblies may be fabricated using a range of pivot options within thespirit of the present disclosure.

FIG. 63 provides an example of a spinal motion preservation assembly3700 with a single pivot design. The spinal motion preservation assembly3700 is shown here without a prosthetic nucleus so that the componentsof the spinal motion preservation assembly can be clearly shown anddiscussed. Distal bone anchor 3704 has a distal cup 3708 that isthreadedly engaged via external threads 3712 on the distal cup 3708 andinternal threads 3716 on the distal anchor 3704. The distal cup 3708 hasa bearing surface for the spherical end 3720 of the single pivot 3724.The proximal end 3728 of the single pivot 3724 is free to move axiallyfor a range of motion within the sleeve 3732 inside proximal bone anchor3736. The movement of the proximal end 3728 is limited by elastomericring 3740 which encircles post 3744.

Sleeve 3732 has external threads 3748 engaged with internal threads 3752in proximal anchor 3736. Sleeve 3732 has a driver engagement section3756 for applying torque from an appropriate driver (not shown). A jamnut 3760 with external threads 3764 and driver engagement section 3768abuts the proximal end of the sleeve 3732 and extends into the driverengagement section 3756 of the sleeve 3732.

Also visible are seal ring 3780 and alignment marks 3784 which are usedto align the two anchors on the driver for timed delivery.

As discussed in connection with FIG. 9, a spinal motion preservationassembly using a single pivot will not have the same range of motion asa similar spinal motion preservation assembly using a dual pivot. Spinalmotion preservation assembly 3700 will have the ability for compressionalong the cephalad/caudal axis based on the design using an elastomericcomponent 3740.

Multiple Bearing Surfaces

FIG. 64 illustrates that a pivot 3804 could engage with a pivot cup 3808such that it effectively has more than one pivot/bearing surface pair asat points of the range of motion of the pivot 3804, different portionsof the pivot 3804 are serving as the point of pivot contact (see portion3812 is engaged while portion 3816 is not in contact with the bearingsurface). The pivot may be axially symmetric as shown from above lookingat pivot 3820 or asymmetric as shown in the cross sectional view fromabove pivot 3830. The selective use of a pivot body/cup surface to formmultiple bearing surfaces allows additional control of the motioncharacteristics of the spinal motion preservation assembly.

Selected teachings of the present disclosure could be implemented withdelivery paths that while caudal to cephalad, do not cross the sacrum.Alternatively, a delivery path which can establish a suitable axialchannel could be used to deliver a spinal motion preservation assemblyin keeping with one or more teachings of this disclosure even if theaxial path was cephalad towards caudal so that caudal became distal forthat delivery process.

One of skill in the art will recognize that some of the alternativeimplementations set forth above are not universally mutually exclusiveand that in some cases additional implementations can be created thatemploy aspects of two or more of the variations described above.Likewise, the present disclosure is not limited to the specific examplesor particular embodiments provided to promote understanding of thevarious teachings of the present disclosure. Moreover, the scope of theclaims which follow covers the range of variations, modifications, andsubstitutes for the components described herein as would be known tothose of skill in the art.

The legal limitations of the scope of the claimed invention are setforth in the claims that follow and extend to cover their legalequivalents. Those unfamiliar with the legal tests for equivalencyshould consult a person registered to practice before the United StatesPatent and Trademark Office.

1. A spinal implant assembly for placement in a motion segmentcomprising two adjacent vertebrae and across an intervertebral spacebetween the two adjacent vertebrae; the spinal implant assemblycomprising: first means for anchoring the spinal implant assembly to themore cephalad of the two adjacent vertebrae; second means for anchoringthe spinal implant assembly to the more caudal of the two adjacentvertebrae, a prosthetic nucleus material placed to at least partiallyfill the intervertebral space; a pivot so that after the spinal implantassembly is anchored to both of the two adjacent vertebrae, thevertebrae can move relative to each other; at least one elastomericcomponent to allow motion of the spinal implant assembly in response tocompressive loading and; a threaded component that can be selectivelymoved along a long axis of the spinal implant assembly to controllablydistribute load between the prosthetic nucleus material and the anchoredspinal implant assembly to the two adjacent vertebrae.
 2. The spinalimplant assembly of claim 1 wherein the spinal implant assembly isadapted to allow selectively adjusting a position of a bearing surfacerelative to one of the two adjacent vertebrae.
 3. A spinal implantassembly for placement in a motion segment comprising two adjacentvertebrae and across an intervertebral space between the two adjacentvertebrae; the spinal implant assembly comprising: first means foranchoring the spinal implant assembly to the more cephalad of the twoadjacent vertebrae; second means for anchoring the spinal implantassembly to the more caudal of the two adjacent vertebrae, a prostheticnucleus material placed to at least partially fill the intervertebralspace; a pivot so that after the spinal implant assembly is anchored toboth of the two adjacent vertebrae, the vertebrae can move relative toeach other; at least one elastomeric component to allow motion of thespinal implant assembly in response to compressive loading and; athreaded component that can be selectively moved to produce adistraction of the two adjacent vertebrae relative to one another. 4.The spinal implant assembly of claim 3 wherein the spinal implantassembly includes a bearing surface associated with the second means foranchoring the spinal implant assembly to the more caudal of the twoadjacent vertebrae.
 5. The spinal implant assembly of claim 3 whereinthe spinal implant assembly includes a bearing surface associated withthe first means for anchoring the spinal implant assembly to the morecephalad of the two adjacent vertebrae.
 6. The spinal implant assemblyof claim 5 wherein the spinal implant assembly includes a second bearingsurface associated with the second means for anchoring the spinalimplant assembly to the more caudal of the two adjacent vertebrae suchthat the spinal implant assembly includes a dual pivot.
 7. The spinalimplant assembly of claim 5 wherein the spinal implant assembly includesa plurality of bearing surfaces.
 8. The spinal implant assembly of claim3 including a pair of cups, with a distal cup located at least partiallyin a cavity in the first means for anchoring, the distal cup receiving adistal end of the pivot, and a proximal cup located at least partiallyin a cavity in the second means for anchoring, the proximal cupreceiving a proximal end of the pivot; and at least one elasticallycompressible component located between a cup and a correspondinganchoring means so that the cup can move relative to the anchoringmeans.
 9. The spinal implant assembly of claim 3 wherein the spinalimplant assembly is configured to not impose any limit to the normalrange of motion in all six degrees of freedom of the motion segment inwhich it is deployed.
 10. The spinal implant assembly of claim 3 whereinthe spinal implant assembly is configured to allow a normal range ofmotion for that motion segment.
 11. The spinal implant assembly of claim3 wherein the spinal implant assembly is adapted to allow at least about0.5 millimeters of elastic compression along a cephalad/caudal axis. 12.The spinal implant assembly of claim 3 wherein the prosthetic nucleusmaterial is placed within the intervertebral space but not within amembrane delivered as part of the spinal implant assembly.
 13. Thespinal implant assembly of claim 12 wherein the intervertebral spacereceived a barrier-sealant to at least partially contain the prostheticnucleus material placed within the intervertebral space but not within amembrane delivered as part of the spinal implant assembly.
 14. Thespinal implant assembly of claim 3 wherein the prosthetic nucleusmaterial is substantially within a membrane delivered as part of thespinal implant assembly.
 15. The spinal implant assembly of claim 3wherein the spinal implant assembly includes: a means for anchoring thespinal implant assembly to a third vertebral body located immediatelyadjacent to and more cephalad than the more cephalad of the firstvertebral body and the second vertebral body, and an additional pivot sothat after the spinal implant assembly is anchored to both of the firstand second vertebral bodies and to the third vertebral body, the thirdvertebral body can move relative to the more cephalad of the first andsecond vertebral bodies in addition to the more cephalad of the firstand second vertebral bodies being able to move relative to the morecaudal of the first and second vertebral bodies.
 16. A spinal implantassembly for placement in a spine, the spinal implant assembly adaptedto distribute load applied to the spinal implant assembly between: amembrane at least partially filled with elastically deformable material;a pivoting support member that can pivot relative to at least one of twosupport anchors implanted in adjacent vertebrae; and at least oneelastically deformable component to enable temporary compression of thespinal implant assembly; the spinal implant assembly: adapted forselective adjustment of an anchored spinal implant assembly to allow achange in an allocation of the load applied to the membrane at leastpartially filled with deformable material and the load applied throughthe support anchors to the adjacent vertebrae; and adapted to allow abearing surface for the pivoting support member to be placed at aboundary between an intervertebral disc space and one of the twoadjacent vertebrae as the bearing surface is substantially aligned witha face of one of the two support anchors for the face of the supportanchor placed in proximity to the intervertebral disc space.
 17. Thespinal implant assembly of claim 16 wherein the pivoting support membercan move relative to both of the two support anchors implanted inadjacent vertebrae.
 18. The spinal implant assembly of claim 16 whereinthe pivoting support member pivots relative to a bearing surface in themore caudal of the two support anchors, the more caudal of the twosupport anchors having external threads to engage the vertebra, and thespinal implant assembly adapted to allow movement of the bearing surfacerelative to the external threads on the more caudal support anchor. 19.The spinal implant assembly of claim 16 wherein the two support anchorsimplanted in adjacent vertebrae include a more caudal support anchorimplanted in a more caudal vertebra and the more caudal support anchoradapted to engage a tool that can prevent the more caudal support anchorfrom moving relative to the more caudal vertebra while the bearingsurface is moved relative to external threads on the more caudal supportanchor.
 20. The spinal implant assembly of claim 16 wherein the spinalimplant assembly further comprises at least one elastomeric componentlocated within at least one of the two support anchors.
 21. A spinalimplant assembly, the spinal implant assembly comprising: a distalanchor assembly for placement at least partially into a first vertebralbody; a proximal anchor assembly for placement at least partially in asecond vertebral body wherein the second vertebral body is adjacent andmore caudal than the first vertebral body, the proximal anchor assemblyhaving a set of external threads for engagement with the secondvertebral body; at least one elastomeric compressible componentpositioned in at least one of the set of the distal anchor assembly andthe proximal anchor assembly; and a membrane placed in an intervertebralspace between the first vertebral body and the second vertebral body,the membrane substantially filled with a deformable material within themembrane which at least partially conforms to a void in theintervertebral space; a support member with: a first pivot surface forcontact with a bearing surface connected to the distal anchor assembly;a second pivot surface for contact with a bearing surface connected tothe proximal anchor assembly; and a middle section between the firstpivot surface and the second pivot surface, at least a portion of themiddle section surrounded by the substantially filled membrane; and ameans for movement during installation of the spinal implant assembly ofthe bearing surface connected to the proximal anchor assembly so thatthe bearing surface moves relative to the set of external threads on theproximal anchor assembly to selectively alter a distribution of loadingbetween the membrane substantially filled with deformable material andthe support member.
 22. The spinal implant assembly of claim 21 whereinthe first vertebral body, the second vertebral body, and theintervertebral body are part of a motion segment and the spinal implantassembly is configured to allow a normal full range of motion for thatmotion segment for flexion of the motion segment, extension of themotion segment, lateral bending of that motion segment in which it isdeployed, and the normal full range of rotation of the spinal motionsegment around a cephalad/caudal axis for the motion segment in which itis deployed.
 23. The spinal implant assembly of claim 22 wherein thespinal implant assembly is adapted to allow substantially the full rangeof motion for reversible compression along a cephalad/caudal axis. 24.The spinal implant assembly of claim 22 wherein the deployed spinalimplant assembly is adapted to allow at least four degrees of bendingin: flexion of the motion segment, extension of the motion segment, andlateral bending of the motion segment, and at least two degrees ofclockwise and two degrees of counterclockwise rotation around acephalad/caudal axis.
 25. The spinal implant assembly of claim 22wherein the spinal implant assembly is adapted to allow at least about0.9 millimeters of elastic compression along a cephalad/caudal axis. 26.The spinal implant assembly of claim 21 wherein the elastomericcompressible component includes an elastomeric material.
 27. The spinalimplant assembly of claim 21 wherein the elastomeric compressiblecomponent includes a spring that is elastically deformable incompression.
 28. A spinal implant assembly for placement in a spine, thespinal implant assembly adapted to distribute load applied to the spinalimplant assembly between: a membrane at least partially filled withelastically deformable material; and a pivoting support member that canpivot relative to at least one of two support anchors implanted inadjacent vertebrae; the spinal implant assembly: adapted for selectiveadjustment of an anchored spinal implant assembly to allow a change inthe allocation of the load applied to the membrane at least partiallyfilled with deformable material and the load applied through the supportanchors to the adjacent vertebrae; and adapted to allow a bearingsurface for the pivoting support member to be placed at a boundarybetween an intervertebral disc space and one of the two adjacentvertebrae as the bearing surface is substantially aligned with a face ofone of the two support anchors for the face of the support anchor placedin proximity to the intervertebral disc space.